Why does diesel engine idling consume to little fuel?

[ericnguyen]

Dear friends:

I've heard that diesel engine idling consumes only about 1/80 of
a similarly sized gasoline engine. I cannot figure out the logic behind this difference in fuel consumption level of diesel and gasoline engines. Could someone explain why?

Best regards,

Eric

[Duke2.6]

Your 1/80 number is way out of bed, but compared to a spark ignition (SI) engine of the same bore and stroke and valvegear (in other words, the same mechanical friction characteristics), a compression ignition (CI) engine could use as little as half the fuel of the SI engine at idle, and there are three primary reasons.

First: a SI engine is throttled, which creates a vacuum. Creating a vacuum takes power and this is referred to as "pumping loss". To illustrate, take a bicyle pump and cycle it rapidly. Your arm will tire fairly quickly from pulling against the slight vacuum that's created as you pull the plunger out. Remove the end of the pump and you could do it forever.

The second reason has to do with mixture requirement. With a CI engine, only enough fuel is injected at idle to create enough torque to overcome mechanical friction torque, and since there is much more air than necessary for this fuel to be completely burned, combustion is very complete. SI engines must always have a homogeneous mixture of fuel and air, and ignition can only occur in a fairly narrow range of air-fuel ratio. Igniting idle mixtures is particularly difficult because of low mixture density and exhaust gas dilution from the prior stroke, so idle mixtures must be fairly rich to prevent misfire. In pre-emission engines, idle mixture was typically about 12.5:1 - about 20 percent richer than stoichiometric, so a lot of unburned fuel went out the exhaust. Modern emission control engines typically run near stoichiometric at idle and usually have less valve overlap than pre-emission control engines creating less exhaust gas residual, so they use less fuel at idle, but the "lean" mixture can cause a lot of idle quality and stability issues.

The third reason is compression ratio. CR is a direct determining factor in thermal efficiency, but with decreasing marginal return - i.e. with each additional increment of compression there is ever less increase in thermal effiency. CI engines are usually built with the minimum CR required for good starting and also to limit mechanical stress in the engine. SI CRs are limited to a lower range due to detonation, and at idle the "effective" CR of a spark igntion engine is only about half the specified value because of throttling - you only start out with about 1/2 atmosphere at the beginning of the compression stroke, so the average pressure during combustion is less, which reduces thermal efficiency. The relatively slow flame propagation at idle in an SI engine also gives more time to transfer heat to the coolant, which is energy lost that might be converted to useful work. The slight amount of fuel injected just before TDC in an idling CI engine is consumed very rapidly and less time is available to transfer heat to the coolant.

CI and SI combustion is fundamentally different. In a SI engine, a spark ignites a (hopefully) homogeneous mixture of fuel and air and the flame front propagates in a predictable manner - what is known as "normal combustion". Detonation is "abnormal combustion". In a CI engine fuel is injected as finely atomized liquid particles into hot air - hot enough to create spontaneous combustion, but the rate of combustion is limited by the rate at which the liquid fuel can evaporate and find air. Diesel engines cannot detonate, but they also can't be run at SI engine mean piston speeds because of the time it takes all the fuel to evaporate, find oxygen, and burn, so they won't make the same specific output as a SI engine unless they are highly boosted.

IC engine textbooks devote chapters to explain the intricacies of combustion - at least one for SI engines and one for CI engines, so what I have discussed leaves out a lot of subtle details.

Duke

[ericnguyen]

Dear Duke:

Thanks a lot for your excellent explanation.

Best regards,

Eric