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

[lrg]

It's because of pumping losses, or more accurately the lack thereof. A gas engine has throttle plates that constrict the intakes when the engine is idling. This is necessary because it is the only way to keep a relatively constant fuel air mixture that is required to get the gasoline vapors to ignite. Much of the work a gas engine does at idle is to suck (or pump) mixture into the cylinders which uses fuel. Diesels have no throttle plates because a diesel will happily burn the fuel(as opposed to explode like on a gas engine) with excess air in the cylinder. Diesels don't worry about a mixture, only having enough air available to burn all the fuel injected into the cylinder. As a result, the intakes on a diesel are wide open all the time and idle is achieved but merely injecting only the small amount of fuel necessary to overcome frictional losses and keep the engine turning over. That makes diesels very efficent....80X more than a gasser? Probably not, but still much more efficient.

[rwthomas1]

Hmmm, I though it had to do with diesels running at variable air/fuel ratios more than pumping losses though that must be a factor.... I believe at idle a diesel runs an A/F ratio of about 125:1 where a gasser is at 14.7:1 all the time, idle or WOT. RT

I know I can idle my car for hours and hours and it barely effects that tankfull's fuel mileage.

[P.E.Haiges]

Fuel must be vaporized and mixed with air to ignite and the mixture has to be rich enough for the fuel molecules to be close enough together for an ignition chain reaction. This is more important in a gasoline engine because a rich enough mixture must be in the tiny area between the electrodes of the spark plug for ignition to start. In a Diesel engine, the ignition can start any place in the fuel mixture.

A Diesel uses less fuel at idle because a Diesel uses a stratified charge. This is where the fuel is richer in the small ignition area and then expands to effect an overall much leaner mixture.

In gasoline engines, the gasoline is mixed with the air before entering the combustion chamber and the total fuel mixture must be even richer in order to ignite at idle. That is why carbureators had a separate idling circuit.

The Honda CVCC engine was a stratified charge gasoline engine. It had 2 carbs one for the rich mixture and one for the leaner mixture. This was done mostly to decrease air polution because it used an overall leaner mixture.

I doubt that a Diesel uses 1/80 of the fuel of a gosoline engine at idle but is is a lot less for the above reasons.

P E H

[leathermang]

"I doubt that a Diesel uses 1/80 of the fuel of a gosoline engine at idle but is is a lot less for the above reasons." --PEH

I have read that figure somewhere legit and may have posted that number.. but don't remember where right now...I had it in hard copy and remember letting others read it....Greg

[leathermang]

Some of these descriptions seem to imply that a gas engine, fuel injected, using a precombustion chamber could match the low consumption of a diesel .... anyone want to tackle that ?


(PS, I have a gasoline Briggs and Stratton all cast iron pre WW2 small engine which has a precombustion chamber... carb model....just in case someone was thinking about taking a run at that not being possible.....LOL)

Could the reasons have to do with the more powerful fuel the diesels burn,,, and the higher compression being able to extract that better ?

[mplafleur]

I've heard that a diesel uses 1/6 the fuel at idle than a gasser.

[Jim Anderson]

Thats why a idling gasser will overheat and a idling diesel will get cold.

[lrg]

RT,
You're right about the gasser using a greater proportion of fuel vs air but keep in mind when the throttle plates on a gasser are closed at idle it effectively lowers the displacement. At idle the total volume of gasses that can be drawn into the gassers cylinder is less so the difference in actual fuel consumption is not as great as the relative fuel/air ratios would imply. At idle the gasser uses less fuel than at higher speeds because less of the mixture is drawn into the cylinder. The mixture remains pretty much the same at high or low RPM. It is the vacuum created by the gasser's closed throttle plates that the engine "works" to overcome. This is clearly not the only cause of greater idling efficency in a diesel but I believe it is easily the biggest.

[TimFreeh]

I don't think the restriction of the throttle plate plays a significant role in idle fuel consumption. My old 220D had a throttle plate and used very little fuel at idle. I remember idling it overnight and not being able to see ANY movement on the fuel gage.

A gas engine needs to maintain a 14 to 1 air/fuel ratio for the mixture to ignite. Any leaner than that and it won't burn - a diesel on the other hand, because any amount of fuel injected into the cylinder burns instantly, can operate at varying fuel/air ratios. fuel/air ratios of 125 to 150/ 1 at idle and down to 18/1 at full power.

Diesels operate with an excess of air in the combustion chamber.

Tim

[ericnguyen]

Wow! Many clear explanations. Thank you very much.

Best regards,

Eric

[mplafleur]

An air/fuel mixture of 14.7:1 (stochiometric) is the mixture at which the combustion of gasoline and air produces the least amount of hydrocarbons. It is not necessesarily the best ignition or power ratio.

[ericnguyen]

Actually the stoichiometric air/fuel ratio of 14.7:1 is just an approximate ratio based of the relative ratios of multiple hydrocarbons (saturated such as alkanes, non-saturated such as alkenes, alkadiene etc., straight and chained hydrocarbon isomers etc.) in gasoline (these ratios are different from different sources).

For a hydrocarbon to be fully burned (its final products must be only carbon dioxide CO2 and water H2O), it must satisfy the following oxidation reaction (I already balanced it):

---------------------------------------------------------------------------
CxHy + (x + y/4)O2 -> xCO2 + (y/2)H2O
---------------------------------------------------------------------------

For example, if a hydrocarbon is an alkane CnH(2n+2) such as
decane (n=10) C10H22: for one mole of C10H22, the number of moles of O2 needed to burned this volume of C10H22 is:

(x+y/4) = 10 + 22/4 = 15.5

Remember that one mole of gas has a volume of 22.4 litres at the standard conditions of 1 atmosphere and 0 degree Celcius), i.e. a
mole ratio of 15.5:1 is equivalent to a volume ratio of 15.5:1.

If an oxygen sensor and mass airflow sensor do not work properly, the ideal stoichiometric conditions mentioned above may not be satisfied, and the exhaust products may have lots of carbon monoxide CO, which means that the oxidation reaction lacks oxygen to completely burn the intermediate product CO into CO2. It is the job of the catalyst to convert it CO into CO2.

For the sake of simplicity, I did not consider nitrogen (N2) in the above discussion.

Eric

[psfred]

The main reason that diesels use so much less fuel at idle is that there is enough atmosphere in there to significantly expand when heated. The vacuum in a gasoline engine means there just isn't much in the cylinder, and all the expansion (and hence available energy) comes from the burning mixture. Most gas engine are significantly richer than 14.7:1 at idle, or they won't run at all, charge density is so low the pressure is so low that the charge won't ignite at 14.7:1.

The 14.7:1 is, assuming typical gasoline BTU and hydrocarbon content, the best for combustion. More fuel gives more power, at the expense of complete combustion and soot, eventually fouling out the plugs. Leaner, and the temp gets too hot in the combustion chamber, so you get pre-ignition and engine death.

Diesels have higher charge density at idle than they do at higer rpm (unless turbocharged), and all the fuel injected at idle burns (unlike the poor combustion in a gas engine), and the heat is transferred to the compressed air in the cylinder. More pressure (from more air), better combustion, and less fuel used. Must have a governor, though....

I assume that diesel normally run a bit leaner than gas engines, but it is possible to add fuel past the ideal mixture -- "turning up" the injection pump used to be quite common, and the sight of a seriously overfueled MACK, White, or Cummins truck, with twin plumes of inky smoke and flames rolling out the stacks could make you think you'd met the devil late at night on the interstate!

A direct injection gasoline engine might be made to mix the two modes, but why bother when you can just buy a nice diese

Peter