If you’ve ever wondered how different diesel and gas are, this will explain it all for you.
At the most basic level, modern car engines work on the principle of four simple strokes: intake, compression, power, and exhaust. Repeat this cycle enough times per minute, and it has the ability to make any gear-head grin. Whether diesel or gasoline powered, the four engine strokes are the same, but there are key differences as to how the strokes are executed.
Engine Operation Differences
For gasoline engines, the intake stroke generally involves pulling air and fuel into the combustion chamber. At this point, a diesel engine is only pulling in air. Next is compression, where both engine types squeeze down the air into a small pocket. Ignition is controlled separately for each fuel type. Gasoline engines use a spark plug to time and start the power stroke. This small arc of electricity ignites the air fuel mixture and the powerful blast forces the piston down, churning out that highly craved horsepower. A diesel engine, on the other hand, only has a pocket full of hot air before the power stroke. As the piston approaches the top of its compression stroke, the air is hot enough from being squeezed down so small that when diesel fuel is injected, it immediately ignites. Thus, the timing of ignition for diesel engines is driven by the fuel injectors. Both engine types function similarly for the exhaust stroke, where a valve opens up and the piston forces the spent fumes out of the cylinder.
While diesel engines continue to build a reputation of struggling with emissions, they actually have highly impressive fuel economy numbers in comparison to their gasoline counterparts. Indeed, diesel engines are significantly more efficient in most cases, especially when the throttle pedal is only partially depressed. Much of this is due to differences in the strokes described above, in combination with the self-ignition temperature. The self-ignition temperature is the temperature at which an air-fuel ratio will combust simply from heat alone. In gasoline engines, it is critically important that the self-ignition temperature is never reached during the compression stroke, as this would cause combustion to occur before the spark plug fires, potentially destroying the engine. As a result, gasoline engines have relatively low compression ratios (the amount the air and fuel is squeezed down during the compression stroke), because compression causes a rise in temperature. As diesel engines do not have fuel in the mixture during the intake stroke, they can compress the air much more, exceeding the diesel fuel’s self-ignition temperature. Higher compression ratios equate to higher efficiency, so diesel engines take advantage of this by injecting fuel into the air after it is compressed.
Another diesel efficiency benefit comes from the lack of a throttle body. When you press the accelerator pedal a gasoline vehicle, this opens a valve in your engine’s intake, allowing more air to enter the engine, and thus create more power. The engine’s computer realizes it needs to inject more fuel for the additional air, but you can think of the accelerator pedal as an “air pedal” in this case. For diesel engines, a throttle valve is not necessary. In this case, the accelerator pedal regulates how much fuel is injected. The throttle body used in gasoline engines acts as a restriction, especially when the throttle is only partially open, and this is part of the reason why diesel vehicles have better fuel economy in low load scenarios.
Air/Fuel Ratio Differences
What allows diesel engines to operate based on how much fuel is injected? One reason is the ability of diesels to operate within a very wide range of air/fuel ratios. Gasoline engines generally operate within a range of about 12 to 18 parts air to 1 part fuel (by mass). Usually, this ratio stays pretty close to 14.7:1, as at this ratio all of the fuel and oxygen is completely used up. A diesel engine, however, generally operates from 18:1 to as high as 70:1, and is capable of operating in super lean ratios. When you press the accelerator pedal in a diesel engine, this drops the air/fuel ratio. Significantly more fuel is injected relative to the air ingested, which increases horsepower. Soot is created when diesel engines operate in low air/fuel ratios, which is why you may see black smoke from diesel trucks when they floor the throttle pedal. Ultimately, while the four major strokes are the same for both diesel and gasoline engines, the finer details reveal the highly unique characteristics of each engine type.
Along with the differences outlined above, there are also major distinctions in how each fuel type is capable of slowing down a vehicle, known as engine braking. For more information, check out the video below.