[Tigers] Tiger Gas Mileage

[e.coiner at cox.net]

---- "Smit wrote: 
> I'll bite: Are diesels actually more thermodynamically efficient than
> gasoline engines?

Are you asking "In theory" or in practice?

The elementary thermodynamic models I learned in college thermo classes were called "Air standard cycles"

The Otto cycle is modeled as 4 thermodynamic processes.
1. Adiabatic compression
2. Constant volume heat addition
3. Adiabatic Expansion.
4. Constant volume heat rejection.

The Diesel cycle is modeled as
1. Adiabatic compression
2. Constant Pressure heat addition
3. Adiabatic Expansion.
4. Constant volume heat rejection.

Both models show that increasing Compression ratio increases thermal efficiciency.
At the same compression ratio the Otto cycle is more thermodynamically efficient than the Diesel cycle.

In the real world a spark ignition engine is limited in CR by the knock and ping properties of the fuel.

Diesels don't have that limit and in fact require much higher CR's to get enough heat to burn the fuel when injected.

In the real world the Diesel is more efficient because it has a higher CR.

Two other factors make a Diesel engine get better mileage.

1. Diesel fuel has more BTU's /lb than gasoline.
2. Diesel engine does not have a throttle plate to choke part throttle air flow.

At part throttle operation the gas engine piston is pulling down against intake manifold vacuum. The takes energy to pull against said vacuum.
That energy is not available to drive the wheels.

No throttle on a diesel, no pumping loss= more mileage.

In summary:  Diesels do better for 3 reasons.

1. Higher compression ratio
2. Higher energy content per gallon
3. No pumping losses from the throttle plates.