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Tuesday, September 15, 2009

Diesel Timing

Maybe you have seen the closed circuit gas pump film clips on reality TV? The vehicle at the gasoline pump suddenly bursts into flames while the owner is refueling. Usually the driver starts pumping gas and then goes back into the car to get something. After sliding across the seat, creating a static buildup within his body, he touches the pump handle again with enough stored static spark energy to light the volatile air/fuel mixture at the filler neck. POOF. Then the understandably panicked operator instinctively, but mistakenly, pulls the nozzle out of the filler neck, spewing fuel all over the vehicle and the ground: he has just re-invented the garden hose flame thrower.

If you have started enough camp fires, you may appreciate how dangerous gasoline is. Yet, with a touch of humility, I tell you that the little lady was not impressed with my Boy Scout skills when I nearly burned down the new backyard shade structure after using the gasoline shortcut in the portable fire pit beneath it. The flames leapt out of the pit for 12 feet or so while I stood at the ready: rum and coke in one hand, garden hose in the other. The lesson here? If you need a water hose in your hand, it may be possible that you haven’t quite thought this through. I suspect that some variation of this statement may end up on my tombstone or as part of the narrative for a posthumous Darwin Award.
Introspection aside, have you noticed that not even one of those fiery film clips ever involve a diesel pump? Never diesel, always gasoline. Why is that simple observation important to tuning a diesel motor? And how can you improve your fuel economy with this knowledge? There are answers.
Diesel does not ignite easily compared to other fuels, not at temperatures at which we are comfortable breathing anyway. This is a key to the fuel’s success in high power applications that also benefit from an additional margin of safety. Perhaps if someone had suggested this to me sooner, I might still have a backyard shade structure.

I Want More

In service-department-originated field complaints. Customer A gets 24 MPG, while Customer B is upset with his 11-MPG results. Among the factors that create the disparity: driving style, trip length, weather, vehicle condition, elevation, fuel energy value… the list is long. Tuning is one of the factors that we can control; to our benefit as we shall see.

LLY Duramax Diesel EFI Live Lope Tune


In a perfect world, diesel timing would be constantly varied according to cylinder pressure feedback and a fancy computer algorithm. That technology is being used today in some developments. Diesel injection timing scheduling is only as good as the hardware accuracy used to measure and control it. It is suspected that this may account for a plus-or-minus five degree discrepancy in desired versus actual timing, as commanded by the PCM (ECU). Without fancy cylinder pressure transducers, we are left to determine this experimentally (guess): trial-and-error custom dynamometer tuning is the usual method. This can often be the answer to improved performance and efficiency and it is well worth the expense. What if I told you that it is possible to reap a 30 percent increase in economy, just with tuning? What if I told you that my 8,000-pound 2500HD Duramax gets 24 MPG on the highway… every day? And what if I also said, you can have it too? 
Figure One – Air intake, and Compression Heating

The Spark is Gone

The concept of compression ignition is a shift for those who are accustomed to thinking spark ignition science. The concepts surrounding diesel tuning are similar but, unlike determining when to energize a spark to time the precise moment that ignition is to begin, the diesel motor relies on a less precise mechanism of compression ignition, also known as auto-ignition.
One of four thermodynamic phases in the diesel cycle, the compression stroke is where fresh air is squeezed, or compressed, in the cylinder. As this happens, it undergoes an isentropic compression, a thermodynamic term that describes what we already know: the air heats up. Forcing the air into a much smaller space makes it hot, really hot. This can be appreciated if we look at these temperatures throughout the compression stroke, shown below on the right.
This compression is governed by the following thermodynamic equations to predict temperature and pressure in the compressed state:

T2 = T1 (V1 /V2)γ-1
P2 = P1 (V1 /V2)γ

V1 /V2 is simply your motor’s compression ratio and T1 and P1 is intake temperature or pressure conditions before piston compression. Gamma (γ) is the ratio of specific heats, a known constant (1.4) for air. The compressed air condition T2 and P2 are calculated and depicted in Table One at various piston stages as the cylinder ascends to the top.
Note that these figures reflect merely the temperature of the air as it is squeezed in the high compression ratio motor. The cylinder air charge can reach 1,100ºF before combustion even begins. After combustion, temperatures can very briefly reach 3,000 to 5,000ºF.
This perfect model assumes no heat mechanism losses to the cylinder walls and no blow-by compression loss, so the actual temperature and pressure will be a bit lower. Note in the 180-degree compression stroke, the first 150 degrees of rotation bring air temperature to 775ºF (yellow highlight), a 675ºF heat up. The final 30 degrees (red highlight) of rotation adds the same amount of heat as the initial 150 degrees, bringing it to a scorching 1,298ºF. This inequity is the result of the piston and rod geometry.

Injection and Ignition

Once the air is compressed and somewhere near top dead center (TDC – bottom row of Table One), the PCM comes along and commands injection, a precisely measured shot of micron-sized diesel mist under extraordinary pressure. The finely atomized liquid diesel fuel enters the hot chamber where it quickly begins evaporating. Only at this point can ignition proceed, since liquid diesel is not combustible (not to be confused with flammable). This fuel, like every fuel, has very specific combustion traits. The duration of the burn varies depending on specific chemical characteristics, properties like Octane, Heptane and Cetane plus macro-physical properties such as atomization quality and cylinder temperature. Even the final air charge temperature affects the burn.
The combustion event is ultra complicated by heaps of dynamic variables and conditions. If the diesel injection timing occurs too early then power will be wasted trying to turn the motor backwards and fuel economy is lost. Too late and the exploding flame will be chasing the retreating piston down the cylinder, exerting little force on it. This is why fuel economy suffers as diesel timing is over-retarded or over-aggressive.
Now look at the columns in Table One again. Look at what happens when the intake temperature go up just 100ºF. Final compression temperature jumps over 300ºF. That pretty much translates into 300ºF higher EGT as well. Now you know what can help reduce EGT. If EGT is a concern, begin with a cool combustion air source; it is the most effective EGT aid.
What if ignition occurs well before the cylinder ascends to the top? For example, some petroleum fuels, like gasoline and ether, can auto-ignite at relatively very low temperatures, under 400ºF. In Table One, that would be 60 BTDC. Let’s be clear, if auto-ignition begins at 60 BTDC, you will blow up the motor as the ignition event tries to turn the cylinder (and engine) backwards. This is extremely dangerous, imparting huge backward and downward forces on the crank, threatening to blow it out the bottom of the truck. If still unclear, the deafening bang you hear in such a situation will clarify it. If you have ever mistakenly driven away from the pumps with a tank full of gasoline, my sympathies. By contrast, diesel fuel requires 800ºF in order to auto-ignite.
Table One also gives us an appreciation for what boost does to both compressed charge temperatures and pressure. The column labeled “100ºF Intake (No Boost)” tops out at 55 atmospheres. The column that adds just 20 PSI (1.4 ATM) of boost (typical) adds 75 ATM to bring TDC head pressures to 130 ATM. Yikes! And remember, creating boost is another compression process that produces heat as a byproduct, so boost typically comes with higher air charge temperatures. So, increasing boost has enormous consequences on ignition timing, not to forget the impact on your head gaskets from the added pressure. This TDC pressure reading may well be the best barometer for determining load on the motor and failure analysis should begin with a look at what is happening with the air charge you are using. What happens here, including induction effects, is the platform for torque production… the genesis of the combustion process.
Hopefully, I am developing an appreciation for the potential damage that can be caused by using too much timing, both with boost increases as well as with intake charge temperatures. You might get away with it for awhile but once you encounter conditions that lead to excess charge heat, poor economy will be the least of your problems. Debris on the road will tell the rest of the story. That might come in the form of a 105ºF day, with your hot air intake seeing 180ºF, waiting in traffic, no cooling airflow for the CAC, heat soaking everything and then… floor the accelerator. That 30 degrees of timing, instead of igniting slowly at 10 BTDC, now ignites quickly at 20 or 25 BTDC, (per the auto-ignition difference from the chart) bending rods or worse, driving the crank right out the bottom, taking bearings and lubrication system components with it. The important point here is that you must monitor intake plenum temperatures if you want to play with performance tuning. It will save you lots of heartache.

Shutter Speed

It will be helpful to understand the amount of time we are talking about when referring to the injection event. The pulse itself is measured in microseconds in a very quick and potent event: the modern common rail injector is subjected to 26,000 PSI. A typical V8 injector pulse required to create 300 HP takes 2,000 microseconds or .0020 seconds. The act of blinking takes 20 times longer. That’s fast. In 2,000 microseconds, the hummingbird’s wings will travel up and down once. If moving at 60 MPH, your car will travel 2.1 inches over the duration of the injection event.
Still, for the purposes of evaluating proper diesel timing such analogies lose punch, so let’s zoom in. It might be useful to know how far the crank and piston travel during the fuel pulse event. Crank angle degrees (CADeg) can be used. Let’s say our 300 HP fuel shot occurs at 3,000 RPM and each revolution represents 360 CADeg. The math is not complicated:

3,000 RPM is 3,000/60 seconds

or 50 revolutions every second. In .0020 seconds, then, the piston goes through .002 X 50 = 0.10 revolutions, or 36 degrees. So our 300 HP, 2,000-microsecond pulse lasts for 36 degrees of crank angle rotation. This is something we can now get our minds around, but remember that is for a specific engine speed, 3,000 RPM only. If the motor spins more slowly, say half that speed at 1,500 RPM, then CADeg is half as well at 18 degrees.
Now look back at the compression schedule. In that same 36-degree injection event, there is a 600 to 700ºF charge temperature increase. As fuel is injected into a cylinder under high pressure, it atomizes into small fog droplets and begins to evaporate as it moves away from the nozzle. Evaporation also means evaporative cooling which counters the compression heating to some extent.
The fuel-air ratio at any point in the cylinder may range from near zero, at a point with no fuel, to infinity, inside the unvaporized fuel droplet. In general, the fuel-air ratio is high near the nozzle tip and low away from it, but because of the complexity of the mixing process, the fuel-air ratio does not change uniformly within the cylinder. Combustion can only occur within a relatively narrow fuel-air ratio range. If the ratio is too low, there is not enough fuel to support combustion; if the ratio is too high, there is not enough air. It is a fact that liquid diesel fuel at regular temperatures is about as flammable as a rainy day. A match dropped into a bucket of diesel is extinguished every time. But if the fuel is heated up to over 130ºF, then the concentration of fuel vapor just above the liquid surface will increase and the flame will ignite the mixture. As they say, do not try this at home. As I mentioned earlier, the same low volatility properties at normal temperatures also make diesel fuel safer from accidental ignition at the fuel pumps.

Three Diesel Combustion Phases

As the fuel vaporizes into the hot compressed air, it starts to oxidize. As more fuel vaporizes and mixes with air, the number and rate of the oxidation reactions increase in a chain reaction, until the end of the
Ignition delay period: ignition occurs at many locations independently and combustion propagates very rapidly in regions which have fuel-air ratios in the combustible range.
This initial combustion after ignition begins is called the
Pre-mixed combustion phase: it consumes about five to 10 percent of the fuel used by the engine at typical full-load operation. At the end of the pre-mixed combustion phase, most of the fuel has yet to be injected or is still in a region that is too rich to burn.
But injection continues and fuel continues to vaporize and mix with air, aided by the heat release and turbulence generated by the initial combustion. This quickly generates more gas with the required fuel-air ratio and combustion continues with exponential growth. This is called the
Diffusion controlled phase or mixing controlled phase of combustion: ideally consumes the remaining fuel. Short duration gas temperatures in this phase will exceed 3,000ºF, though the cylinder itself will never be this hot.

Ignition Versus Diesel Injection Timing

So far we have tried to understand how diesel fuel behaves and the importance of heat to diesel combustion. Before proceeding, let’s make a clear distinction between the timing of the fuel pulse and what is meant by the term ignition. They are not the same. If I refer to ignition, I am referring to an actual combustion event. Diesel injection timing must be timed in response to the factors affecting ignition quality and timing. Normally the ignition event closely follows the beginning of the injection event. Since they are related in time, understanding ignition is the first step toward determining the necessarily correct optimized injection timing. It may seem exhaustive to emphasize this, but we tend to use these terms interchangeably and that can lead to confusion as we try to understand what is happening inside the cylinder.
A small liquid droplet of diesel alone does not ignite. Only evaporated fuel vapor is combustible. A combustible mixture of oxygen (air) and evaporated fuel vapor must exist in an environment hot enough to auto-ignite it. For ignition to then occur there must be very specific proportion of fuel vapor to oxygen, a narrow range of proportions, or there will be no explosion. To make matters more complicated, even these proportions change with changes in temperature. These proportions also affect ignition quality. This was briefly discussed earlier.
In our quest for best fuel mileage, we have to adjust injection timing against the collective ignition characteristic of the fuel and engine properties

Optimum Diesel Timing

So what constitutes optimum timing? What model will help us make a diesel timing chart? Design engineers have many objectives, trying to satisfy several authorities, while adhering to government regulated restrictions. It is their desire to obtain maximum torque, acceptable EGT, minimal noise emissions, maximized economy and lowest chemical emissions, all the while attempting to maximize the life of the motor. It is a pure juggling act of trade-offs and it is rare when any adjustment can be made without some other negative consequence. For example, retarding cruise timing lowers particulate emissions and noise, but decreases economy. So how is timing determined in the cylinder environment that has such dynamic thermal conditions? From Alaskan Permafrost Winters to Death Valley’s waterless Summers, timing must be adjusted constantly. For optimal diesel timing, it is essential that the onset of ignition occurs somewhere around the top of the compression stroke. Again, if the mixture auto-ignites too soon – before the piston reaches TDC – then much of the work of early combustion will try to turn the crank in reverse, as the piston is ascending against the expanding gas combustion product. This negative torque is obviously harmful to economy: it is the biggest inefficiency that the internal combustion engine faces. If injection is too late, then the combustion product flame front will be chasing the quickly receding piston down the cylinder and little torque will result. Poor economy again. To maximize economy, the combustion event must occur to minimize negative torque and maximize positive torque. If ignition begins at 28 degrees before top dead center, the piston tries to go in reverse for 28 degrees. If you start injection so that ignition begins at 10 degrees before top dead center then it is only acting against it for 10 degrees. The engine becomes more mechanically efficient with increased, net positive torque. Of course the piston resists reverse motion because of the momentum served up by seven other cylinders that are not at TDC. There is always a component of negative work before the more abundant positive work dominates.
Figure Three – Cylinder pressure effects of too much timing

 In Figure Three, the effects of over-advance begin to become apparent. A 40 percent increase in cylinder pressure accompanies a 20 percent decrease in power. Immediate extra fuel costs and eventually shortened life span results. Figure Three shows that cylinder pressure peaks after TDC. Brake mean effective (cylinder) pressure (BMEP) or sometimes IMEP is used. These terms are similar and are defined as the average pressure that the entire torque stroke would have to exert to equal the torque produced by actual net positive pressure.

What we know as torque or net positive torque is determined from the total area under the curve after the TDC mark and subtracting the area under the curve before TDC. The integration of these is net positive torque.

Piston Geometry

Becoming familiar with the 360 degree piston cycle, I remember thinking; the pure geometry of the torque down stroke suggests that 80 ATDC is the place where you want to have all your work performed, instantaneously. The only problem is that at 80 ATDC everything is moving at its highest velocity away from the chasing explosion. At 80 ATDC, the cylinder speed largely outpaces the flame front velocity. The other less obvious problem is that as the cylinder progresses past TDC, expanding cylinder volume, without an ignition event, temperature begins to drop, in reverse of what we saw in the compression schedule: and it falls off fast, in fact, you risk a complete auto-ignition miss.

Fuel Mileage Equals LPP

Let’s fast forward through all the experimentation: using an in-cylinder pressure transducer, it has already been determined that the maximum torque is obtained when the location of peak cylinder pressure, LPP, occurs between 10 and 16 degrees ATDC. The standard reference optimum LPP for internal combustion motors is 12 or 14 ATDC, depending on the referenced study. I will use 14. If ignition initiates at its optimum timing, 15 degrees BTDC (a hypothetical number), the burn would progress through the chamber and peak pressure would occur at 14 degrees ATDC. To maximize economy, diesel injection timing is adjusted by the engineer to make good the desired LPP.
The fuel pulse is necessarily of finite duration and the time that the injector is open can span 40 or more degrees of rotation, a small window of time. Combustion is not finished when fuel injection stops; it continues as long as fuel vapors find oxygen. This is called after-burning and it lowers the efficiency and performance of the engine because the cylinder volume is increasing, well past TDC. As the fuel quantity per cycle is increased, after-burning increases and results in incomplete combustion at the end of the expansion stroke, although air is still present. This implies the importance of good atomization. Poor injector condition or low fuel rail pressure leads to after-burning, poor economy and smoke.
Figure Four – Fuel Quality Impact on Peak Pressure and Combustion Duration

Fuel Quality, Cetane, Ignition Delay

Diesel ignition quality depends on two main characteristics that the power plant designer is concerned about: ignition delay and combustion duration. During normal operation,
Ignition delay is the single biggest variable to accurately choosing an injection. This one fuel property can change dramatically depending on conditions, as low as five CADeg to as much as 60 CADeg. It presents the biggest challenge to emissions as well; combustion completeness is very much dependent on correct diesel timing. Delay is primarily governed by chemical properties, though when the motor is cold, the physical evaporation delay is also significant.
Combustion duration is governed by chemical properties and the environmental factors, temperature and pressure.
Cetane Number is a measure of how shortly after start of injection the fuel begins to auto-ignite. The engine requires an increasingly higher cetane number fuel to start easily in the cold. To change cetane content is to change ignition delay. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.
Noise level is affected by cetane content. In the United States, the ASTM Standard Specification D 975 cetane number requirement is a minimum of 40. Consequently, as a culture, we have louder vehicles than Europe where the specification is mandated to a minimum 45. The source of the noise results from the rapidness of the pressure rise. If fuel has a long ignition delay period, then it finally ignites later when the charge temperature is considerably hotter. The hotter environment accelerates ignition. The quickness of this event is the source of the typical clatter we here. When ignition begins sooner, with less delay, the pressure rise is more gradual and quieter. The difference can amount to several decibels – as I have found.
Figure Four shows the ignition delay and cylinder pressure reduction with Cetane. The higher cetane rating results in lower cylinder pressure, yet more net positive torque… win-win. Notice that LPP is the same in each, controlled to 18 ATDC in each case.

A Rough Start

My truck smokes on startup on cold mornings, what is wrong with it?
Nothing. This is called cold smoke and is always white. It is unspent fuel mist that emerges with the spent exhaust. It usually will only last a minute or less, dissipating as soon as the cylinder wall skin gets warm enough to efficiently sustain auto-ignition. Until the temperature of the cylinder and piston is warm enough to maintain the temperature of the compressed charge, power will be down, efficiency will be low, and the motor will be louder. If you have numerous short trips, plan on poor economy.
The heat of compression is responsible for heating the vapor in the combustion chamber to a temperature that will initiate the spontaneous combustion of the fuel. When a cold diesel engine is started, the walls of the combustion chamber are initially cold, making them heat absorbers. Since cranking speed is slower than operating speed, compression is also slower; this allows more time for the compressed air to lose heat to the cold chamber walls or through gradual depressurization (expansion equals cooling) as air leaks by the rings/valves. Adding to the cold start heat deficit, the fuel pulse is evaporating, cooling the charge significantly and worsening the cold start dilemma: a conspiracy of starting impediments when combined with cold weather. As such, an additional localized source of heat for cold starting indirect-injection diesel engines is needed, a glow plug. Even after the engine start, the temperatures in the combustion chamber will still be too low to produce complete combustion of the injected fuel. That white smoke you get for the next minute or two is called cold smoke. It is diesel vapor mist that does not get burned because the cylinder is not hot enough. It is an awful eye and respiratory irritant as well.
A fuel that combusts more readily will require less cranking to start an engine. So, if other conditions are equal, a higher cetane number fuel makes starting easier. Incidentally, cetane and octane are, in a real sense, inversely related. Increasing octane increases ignition delay. The same chemical characteristics that make for good diesel combustion amount to poor detonation resistance in gasoline: high octane equals low cetane and vice versa.

Optimization Concepts

Let’s assume an auto-ignition temperature of 800ºF for our diesel platform. With a fixed 15 BTDC injection pulse, the ignition event begins at around 10 degrees BTDC, using a standard cetane delay value of five degrees. If the ignition event lasts for 40 degrees of rotation, then there is 10 degrees of negative torque and 30 degrees of positive torque resulting in 20 degrees of net positive torque.
But what happens if the entry air charge heats up and is now 240ºF? This is an actual example of the effect of higher load and induction heat soak while climbing a grade. If the timing event is unchanged then there is 13 degrees of negative torque and 22 degrees of positive torque, net positive torque has been dramatically reduced over half to only nine degrees. Note the loss of five degrees of duration; this is due to the decrease in combustion time that results from the hotter environment. So to restore net torque to 20 degrees, diesel injection timing must be retarded five degrees (five degrees has a 10-degree net effect, a little change goes a long way). If this adjustment is not made dynamically, then you experience an efficiency reduction and a noticeable loss of power on grade.

Diesel Timing Damage

Often added timing results in better fuel economy. Many people want to know how much advance they can get away with to enhance performance. Unlike the spark ignition gasoline motor in which detonation is the timing advance limit, the diesel motor can be advanced without the immediate threat of short term detonation damage: the diesel componentry is typically beefed up to handle the added stress. How does this work? Say you advance injection to a ridiculous 45 degrees... very shortly after injection there is a combustible air-fuel mixture, but the environment is not yet hot enough for a quickly progressing auto-ignition event. In fact, the premature injection and subsequent evaporation adversely cools the compressed charge. As TDC approaches and the ignition temperature is finally reached, a very saturated air-fuel gas mixture creates a very quick burn and the fast moving pressure front creates more noise, the signature diesel rattle. The result is some very high cylinder pressures before TDC, less net positive torque and the potential for serious motor damage, particularly if thermal conditions worsen, as we shall see next.

Fire in the Hole

What if you are towing with this radical 45-degree advance and you sustain a high load for a minute or two on a grade? ECT rises, the cylinder gets progressively hotter. IAT rises, so the air charge compression schedule is hotter now, the auto-ignition event occurs more rapidly at an earlier point in the stroke, say around 30 BTDC. This saturated environment abruptly ignites, quickly releasing most of its potential well before TDC and the result is like a grenade in a milk can, resulting in catastrophic cylinder pressure and detonation, much like what happens with over-advance spark engines. If the advance is enough, this will lift the heads or destroy the motor within seconds, without much audible warning whatsoever. It is an insidious killer. When you examine the piston-rod geometry and the forces acting on these members, you will see that all this energy is trying to drive the crank out the bottom of the motor and bent rods frequently result. Burned pistons can also develop, the result of the much higher temperatures and pressures. Also important is the realized loss of power on grade, resulting from reduced net positive torque. This is a key observation. If you notice that as your motor heats up, that you are beginning to suffer a power loss, it is trying to tell you that it has entered this over-advanced state of stress. To the casual operator, it is counter-intuitive: it is hard to believe that the engine could blow as it is producing less power.
Fortunately, the engineers have a way to resolve this issue using a PCM strategy that reduces timing under factors that tend to advance ignition. If you are doing your own modified tuning and you want to be aggressive with advance, it is important to become familiar with these concepts and their affects on ignition. At a minimum, remember to safeguard against this kind of stress with timing retard conditional upon ECT and IAT increase. Most PCMs now have this capability, and for GM, nothing makes this customization possible like EFILive.

Factors that Affect Ignition Delay and Duration (and Therefore Affect Required Diesel Timing)

These are presented in order of importance, the first ones having the most impact on ignition. Ultimately, the issue at hand is the affect each factor has on combustion duration. If a change shortens combustion time (as most of these do) the diesel injection timing must necessarily be retarded, and vice versa. Simple enough?
The first set of factors are under the operator’s control…
Figure Five – Heating/cooling affect of the air’s compression/decompression.

RPM

RPM changes the amount of time that the fuel has to evaporate. A faster moving piston requires more timing advance because it arrives at LPP in less time than a slower moving piston. To prepare fuel for combustion at the same crank angle position, it must be injected sooner to have the same amount of time to be combustible near TDC. Looking at the timing curve, the trend is to advance timing as RPM increases.
Load
The term load is commonly misunderstood and misused. Load literally refers to the forces pushing back against the piston as it tries to work. Headwind, grade, downshifts all represent increases in load. Here we will define it as how much pressure exists in the cylinder at LPP. As load increases, exhaust gas temperature increases with these higher pressures and richer engine operation. With more energy to be recovered in the turbocharger, the resulting boost pressure increases. Accordingly, cylinder pressure and temperature during the ignition delay period increases. Higher pressure and temperature with higher load eventually decrease the ignition delay period. Ignition delay can range from three CADeg under high load to as much as eight CADeg during cruise. This suggests the need for actively retarding injection for higher load. All these factors try to slow the vehicle. Your response is to deliver more fuel using the right foot. This is the primary metric we use to set diesel timing for load changes: more load > less advance.
Higher load normally requires a longer fuel pulse, 200 or even 300 percent longer duration than a low load pulse. Naturally, that requires an advance to keep LPP in position, so the effect of load, and the longer pulse that is associated with load, have a tendency to cancel each other out. But the net result is a small ignition retard, requiring modest advance. That may not ring quite clearly, but what is clear is that RPM increase alone, reduces load. Combined with greater fuel requirement means timing must be advanced.
Another tidbit worth mentioning, as load increases, pre-mix combustion duration becomes smaller and smaller, yielding to diffusion controlled combustion. This helps explain why low RPM, high fuel events require very low timing numbers.
These two factors, RPM and load, explain most of the trend in the stock Duramax base timing chart (Figure Six), compliments of EFILive. This table is a custom table I created for a specific custom Duramax towing application. The trend to advance timing with increasing RPM is most obvious.

Environmental Factors

These factors are, for the most part, outside of our control...

Altitude

Elevation changes air density more than any other factor. There is 40 percent less oxygen at 10,000 feet than at sea level. This reduction in oxygen makes for a longer ignition event and diesel injection timing must therefore be advanced to maintain a correct LPP. This effect can be partially offset with turbo charging, analogous to how pressurizing the aircraft cabin keeps us conscious at 40,000 feet. Increase altitude > advance timing.
Figure Six – Timing table depicting dependence on RPM and fuel

Intake Air Temperature

IAT is important because the temperature of the compressed cylinder greatly affects ignition delay and the speed of the ignition event. Warmer air speeds up fuel evaporation, reduces ignition delay and combustion duration so diesel injection timing must be retarded. If the PCM does not account for this, then the motor will find itself losing power with non-optimum, over-advanced ignition. This is especially true if IAT increases with the increase in vehicle effort as it does in a thermal feedback relationship. Increase IAT > retard timing.

Humidity

This one is very frequently overlooked and to the best of my knowledge there is no PCM correction for it on any vehicle that I am aware of. Usually tuning is set with humidity at some average value. But the impact of humidity on ignition is not insignificant. Increased humidity has a pronounced ignition retarding affect, requiring some advance to compensate. When fuel is combined with wet air, the combustion event progresses at a smooth rate, moderated by the water molecules that exist in gas form. It does in a real sense, slow and moderate combustion which can be a benefit to torque production. Hot dry air, void of this assist, will ignite sooner, burn faster, and with less total torque in each stroke.
Contrary to a popular misconception, humidity does make the air lighter , that is, less dense, since water molecules weighs less than the oxygen and nitrogen molecules in air. Water does indeed displace oxygen, but only minimally. However, the cooling effect of humidity is known to be a big benefit to the EGT alarms because the water vapor has higher heat carrying capacity than air and therefore the cylinder does not get as hot. Increase humidity > advance timing.

Boost and Compression Ratio

These factors affect ignition much like load does. Increasing them creates a more oxygen-rich environment and, in most cases, a warmer, more reactive one. Faster ignition results (creating ignition advance), which necessitates retard to moderate it.
Tests have determined that air density itself has little effect on ignition delay, and this fact is often misstated. The change in ignition delay is normally due to the changes in temperature or pressure or both that accompany the density increase. Remember also Table One. The huge increase in cylinder pressure that results from boost increase, removes margin as we approach maximum permissible head pressure. This mandates close diesel timing scrutiny and requires conservative measures to keep from blowing the head gaskets and destroying piston components. This is not the time to squeak out that last 10 HP with two more degrees of timing advance. Boost plus over-advance equals disaster at high fueling levels. Increase boost > retard timing.

Fuel Temperature

This factor affects fuel viscosity, injector atomization and evaporation rate. Diesel at 150ºF will atomize into drops half the size of diesel at -30ºF. This also means twice the liquid surface area initially exposed to the charge air. Evaporation rate speeds up leading to a faster combustible mixture. So fuel that is warmer has ignition that proceeds faster, requiring injection timing retard. Increase fuel temperature > retard timing.

Fuel Quality

This normally refers to cetane rating. A science unto itself, a higher cetane rating results in a more consistent ignition delay and, as we have seen, noise reduction. A higher cetane rating allows the tuner to retard timing slightly to maintain the same LPP. The noise reduction is well worth the effort. Increase cetane > retard timing.

Practical Application

Now let’s assume that you are towing a grade in the desert Southwest at 7,000 feet; outside temperature is 92ºF with six percent humidity. Assume further that you are using a boost enhancing device in order to offset the high EGT associated with these conditions. Soon the engine is heat soaked at 230ºF ECT and the air charge leaving the intercooler (IAT2) has crept up to 240ºF. And you are now looking around asking why is this truck continuing to lose power?
Now you know the answer. Besides having thin air at that temperature, the stock injection timing is too advanced for the conditions and too much of the fuel is trying to turn the motor in reverse. If you take away five degrees of timing advance, say from 12 BTDC to seven, using a handy switched EFILive tune, some of that torque and power will be instantly restored on grade. If we are burning the same quantity of fuel, then more power equals more speed and that means better fuel economy.
Another option is to remove boost. “Wait!” you say. Won’t that reduce my power also? Not exactly. Since more boost creates faster ignition and vice versa, if we remove some boost from an over-advanced scenario (by virtue of heat) it is like pulling timing. This would be an option to help correct the runaway IAT condition seen in some vehicles. Without our even being aware of it, this is what a wastegate does, reducing boost as induction heat conditions worsen. For the VGT owner the wastegate is not available, so I have developed algorithms to dynamically position and maintain LPP in changing conditions where the above factors want to skew our optimized platform.

Dynamometer Limitations

Take another example, a dynamometer-tuned performance vehicle. Usually, dyno tuning does not take into consideration the gradually changing conditions the vehicle ends up experiencing in the real world, especially torque-rich work vehicles. After it comes off the dyno, you head out to the track, and do three sequential back to back runs. Maybe each quarter mile time drops slightly from the one before. Why? As the day progresses, the track is getting warmer, the motor is also. The CAC is getting heat soaked with each pass, and oddly, IAT (if you are monitoring it) is getting higher and higher because your lousy hot air intake is pulling ever-warming air from the hot engine.

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