fuel ecomeny

[Mhannink]

ok im a newbie here tho ive been around diesels and cars for a while and i understand internal combustion engines and a few others and will be goign to wyo tech when im out of highschool so horsepower is my thing the one thing i know about but don't understand is how a diesel can have 500 hp and get 20mpg while an equivliant gas gets next to nothing the one theroy i have on it has to do w/ the fact that the diesel is governed by the fuel/fuel pump and only sprays just enough fuel to maintian rpm in turn this makes for very little comsuption and since power is made by efficiecy as much as amount of fuel and air the engine use just that little bit to run down the road while it can pour it on to make lots of power the other side of this is that the gasser is throttled by the incomeing air and then mixes the proper amount of fuel for the amount of air always maintainging the same mixture ratio another thing i do think the direct injection of a diesel has a hand in its effeicy along w/ have nealry 2 the poteintal energy of the same amount of gasoiline anyways tell me where im wrong and please correct me im trying to learn here thanx in advanced

 

[BarryG]

If I understand what you are saying you are right there in the ball park. The term is schoichiometric. If I recall gas has a shcoichiometric rate of around 14:1 or so more or less at all times. Diesel's is typically higher and can be much higher like 100:1 at idle. Do a google search on the term and surely a bunch of stuff will come up. Also diesel has more btu's per gallon.



Am sure someone with a much better physics background than myself will chime in and correct me if I am wrong and I may be.

 

[jwgary]

There's really lots of things going on here. The incoming air in a gasser is throttled meaning you open or close a restriction in the airway. There is always a near stoichiometric mixture of fuel and air meaning that you have just enough air to burn the amount of fuel in the air. Gasoline engines also can't run much more than 10:1 compression ratio or else the fuel in the air will autoignite (knock).



While you are running down the road you have the throttle barely open; just enough to keep you running at a constant speed. This means that the air must flow around the butterfly and you get pumping losses as the intake stroke causes a vacuum in the intake manifold. A diesel engine is not throttled so you don't have the pumping losses.



With the diesel engine running air/fuel ratios of around 50:1 at cruise, you are using less fuel for each charge of air. Of course that really about evens out since you need the same amount of energy input for the same energy output. For certain cases though the diesel has better efficiency here too.



The main advantage is the compression ratio. Since a diesel usually runs at least 16:1 or greater it has the advantage. An engines volumetric efficiency is directly related to compression ratio and the higher Rc you have the better the efficiency.



Other things like combustion temperature help out in the mix as well. Hope that helps a little.