Texaschevelle

I just don't get it! I just got into the diesel thing and i just don't understand how a diesel engine can go up on horsepower and TRQ, and still get better fuel mileage. Being i have only dealt with gasoline engines in mild to moderate performance, it has always been know that you force more fuel into the combustion chamber, burn it, and make more power. Can someone explain to me how this works in a diesel. If you gain 100 hp in a gasoline engine, let's say you lose some drivability and 4-5 MPG. In a diesel you may gain that. To quote Jerry Seinfeild, "What's up with that?"

Box5

with the right mods it will burn the fuel more efficiently. And especially with a turbo diesel, it has alot to do with the air u can push, and also that diesel is more dense and has more energy. Plus once u get the right air flow, ur burning the fuel more efficiently because the air is already heated before the fuel gets there. And the higher compression of a diesel also lets it produce better then gas engines..

wcbcruzer

Well it all depends on your right foot. Your fuel mileage will go way down if you constantly use the added 100 hp. It all depends on the type of modding you do. Increasing the timing a little and getting more efficient (and more powerful) injectors will increase fuel economy IF you drive it good. To add 100 horsepower on a gasser, you normally have to modify the engine a lot from factory settings, which tunes the engine so that there is always a lot of fuel going in. On a diesel, you don't modify it much at all; so you can control how much fuel goes in at will.

HOHN

Good question to ask.

The difference is in the fundamental difference between diesel and gas engines.

A diesel can go up in POTENTIAL hp and tq, and delivery POTENTIALLY better mpg-- but it can't do both at the same time!!

If you think about it, that's the beauty of the diesel. It can be either a hot rod, fuel guzzling engine when you want it, or it can be rather efficient if you want it to.

Contrast this with a gas engine, where most of the time a hi-performance gas engine gets worse economy whether you're into the loud pedal or not. Whether it's the bigger carb, high-overlap cam, or something else, the hot rod tricks used on old-school muscle car V-8s give worse mpg.

Also keep in mind the variable fuel ratio of a diesel, and the fixed ratio of a gas engine. Diesels by nature run very lean. Hence, when you crank up the power, the run a little "less lean" but still run clean and efficiently.

Compression ratio matters, too. How many gas engines have you seen running 17:1 compression? It's not so much the high compression as it is the corresponding EXPANSION ratio. The diesel's high expansion ratio extracts more usable work from a given amount of fuel (Carnot efficiencies, etc).

The diesel also lacks a throttle plate, which means it doesn't have the parasitic "sucking" losses a gas engine would have.


Interestingly enough, GDI (gasoline direct injection) is showing that you can use the diesel process to gain similar improvements with a gas engine. In these engines, the injector is in the cylinder (just like our CTDs) and the stratified charge burns very lean. As a result, they can run very high compression with no detonation (no fuel until just before the spark).

Essentially, it's a diesel engine using spark ignition to light gasoline, so it's sort of a "hybrid" in that sense.

jh