Diesel Engine 101

Diesel Articles - General

Well, folks, diesel engines are here to stay. Virtually every carmaker has plans for a diesel engine either in their pickups, SUVs or passenger cars. Its only a matter short time before we will see diesel pumps added to gas stations that previously did not offer diesel fuel. Now Audi has started a TV ad campaign which shows a young woman filing her Audi diesel powered car at the filling station. Well-meaning fellow motorists are shouting to her things like: “Hey! That’s diesel fuel!” or “Stop it right now before you do harm to your car!” to which she responds basically; “Relax everyone, I know what I’m doing.”

This article is meant as a refresher course to those of you who consider yourselves “Diesel Savvy” and and intro to those of you who are new to the diesel power scene. Read on.

Major differences between the gasoline engine and the diesel engine are:


  • Gasoline engines take in a mixture of gas and air, compress it, and then ignite the mixture with a timed electrical spark.
  • Diesel engines take in only air, compress it to super-heated temperature and then inject fuel into the super-heated compressed air. It is the heat of the compressed air that ignites the fuel spontaneously, generating the downward power stroke of the piston.

Compression ratio

  • Gasoline engines compress air/fuel at a ratio of 8:1 to 12:1 (on average)
  • Diesel engines compress at a ratio of 14:1 (and as high as 25:1). The higher the compression ratio of the diesel engine, the better the combustion and fuel efficiency.

Fuel Delivery

  • Gasoline engines generally use one of three fuel delivery methods. The first is carburetion, which pre-mixes the air fuel mixture at the top of the intake manifold, far from the cylinders. Second is direct electronic fuel injection, in which the air/fuel mixture is injected into a port, before the cylinder at the beginning of the downward travel of the piston on the intake stroke. Finally, there’s indirect injection, where the fuel injector is located away from the cylinder and the air/fuel mix travels to the cylinder via intake plenums.
  • Diesel engines use direct fuel injection into the combustion chamber. Diesel fuel is injected directly into the engine’s cylinder when the piston is at the height of travel (TDC) of the compression stroke and the chamber is ready to ignite the fuel and initiate the power stroke.

The Cold-Start Process

  • Gasoline engines use an extremely rich air/fuel mixture (which is highly volatile in the presence of an ignition source) to achieve cold starting.
  • When a diesel engine is cold, the compression stroke may not raise the air to a temperature hot enough to ignite the fuel; therefore a glow plug or other means are used to prepare the combustion chamber for ignition. A glow plug is an electrically heated device that helps ignite the fuel when the engine is cold. On diesel engines that have an ECM, (Engine Control Module) the ECM reads air temperatures and if it senses that its cold outside, it retards engine timing and sprays fuel at a later time than at TDC. This adjustment makes for greater compression of air, raising combustion chamber temps and making cold starting easier. Additionally, block heaters are used to keep the engine’s cooling system warm, preventing the oil in the crankcase from gelling up and creating higher resistance to the crankshaft as it turns and ensuring the engine cranks fast enough to start. Finally, diesel fuel during cold weather tends to gel up, creating resistance to flow, anti-gel compound ensures the fuel does not stop flowing through the delivery system. Anti-gelling compound is added to the fuel tank.

The Difference Between the Fuels

When crude oil is processed at refineries, it is separated into several different kinds of fuels, including gasoline, jet fuel, kerosene and diesel. Diesel fuel is heavier and oilier than gasoline. Plus, diesel fuel has a higher energy density than gasoline.

Here’s a quick thermodynamics lesson to illustrate this fact:

  • 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules or 147,000 BTU
  • 1 gallon of gasoline contains 132x106 joules or 125,000 BTU

In short, you get more bang for your buck out of a gallon of diesel fuel (about 17 - 25% greater efficiency) than you do from a gallon of gasoline (this is a result of the steady, even and complete burn of the diesel fuel as it expands and pushes the piston downward in the power stroke).

Then There’s the Danger of Mis-Fueling Your Diesel Vehicle

What happens if you inadvertently fill your diesel car with gasoline? Whether you're new to diesel ownership, or might own both diesel and gasoline powered vehicles, it can be easy to accidentally mis-fuel your diesel tank with gasoline. Filling your fuel tank is such an ordinary task, that just a moment's inattention can cause you to grab the wrong nozzle and pump.

Hopefully you realize the mistake right away and get the car towed to a shop to have the tank drained and the system blown free of the offending fuel. But what if you don't even realize the mistake and end up driving away with a tank full of gasoline? Chances are you won't get very far (maybe just a mile or so until the diesel in the fuel line gives way to the fresh batch of gasoline on the way from the tank) and the engine starts to run "funny."

Granted, it all depends on how much diesel remained in the tank before the gasoline was added and how new and sophisticated the diesel engine in your car is. For the record, in an ‘07 or newer clean diesel engine, any amount of gasoline will probably damage the sensitive emission control components and/or system. In older engines with much less sophisticated and "touchy" emissions systems, a lightly diluted (say 90 percent diesel/10 percent gasoline) mix would probably pass through with little or no detriment (hopefully that is…). It might simply cause reduced engine power, perhaps a bit more noise, and possibly a sharp warning from the emissions sensors that detect something other than pure diesel exhaust. It's high concentrations of gasoline that spell real trouble in a diesel engine. Whether a modern clean running diesel or an old indirect injection unit, burning straight gasoline or highly diluted diesel fuel will almost certainly result in catastrophic damage to the engine. If you mis-fuel your gasoline vehicle with diesel fuel, it will start running poorly and stall, get it to a shop, drain and clean the gas tank, blow the system out with air and hopefully it will start and run without fouling any sensors for the emissions system. In most cases, this is all you’ll find wrong with the engine.

That does it for this week. Diesel Class dismissed!

‘Til next time…Keep Rollin’


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