Weight is only relevant to acceleration (which is partially recovered), hills (partially recovered), and rolling resistance which is only ~5% of energy usage. Even with a consistent load on the highway the batteries can help by removing parasitic loads on the engine like alternators, a/c compressors, and power steering pumps. Depending on torque demand and specifics of the engine/transmission, thrashing the battery could also be more efficient than ICE alone.
I'm sure there's stuff I've missed, but I think those are the main reasons why a number of hybrids have highway EPA ratings 10-15 mpg higher than the highest MPG ICE car ever produced.
you're totally right, I mixed up my facts. they are 10-15 mpg higher than the highest MPG ice car currently produced for the US market which is the Mitsubishi mirage.
Not quite. Diesel engines are most efficient (~40%) at full throttle, low-mid revs, and drop to <30% at low throttle. With some battery trashing you could see the engine work at some 20-50% duty cycle on the highway.
Not quite. otto cycle is most efficient at full throttle due to throttle valve pump losses.
Diesels are most thermodynamically efficient at low throttle because it takes time to combust the fuel. During this time the piston expands and the compression ratio drops.
You can see the derivation and formulas in Faires (and older edition. Even the 70s edition lacked the chapter). Actually just comparing the the PV diagram of otto and diesel makes it obvious.
First a nitpick that is unfortunately important - diesels' don't have a throttle; what we call throttle is the fuel cutoff ratio, or how long we inject fuel for.
This is important because what distinguishes Otto and Diesel cycles is not the fuel, but the form of ignition. Otto cycle analysis is assumed instant, whereas diesel cannot be assumed instant. Car otto engines are fast enough that a diesel-like cycle is used for advanced analysis.
Low rpm and low throttle improve diesel's efficiency for the same reason - the injected fuel has more time to burn without the piston expanding.
As [1] points out in page 180:
"Study of equation (62) shows that as r e increases, the bracketed factor increases, and the efficiency decreases (Fig 99) Therefore, the lower fuel cutoff ratios are conducive to higher efficiencies but larger ratios result m greater power "
Also, very interesting, Diesel's efficiency is lower than Otto all else being equal. However, due to knocking, gasoline (octane ~ 90) engines have low compression ratios and lower efficiency than Diesels. However an otto methane (octane of 130) engine would kick diesels' butts
Diesels running constant RPM will increase throttle with increasing load to keep their RPM. That's a matter of injecting more fuel. It's how every diesel based generator works.
As for high octane kicking diesel's butts: yes, but that fuel is far more expensive so it tends to be diesel that wins that particular contest in the economy department.
Weight is only relevant to acceleration (which is partially recovered), hills (partially recovered), and rolling resistance which is only ~5% of energy usage. Even with a consistent load on the highway the batteries can help by removing parasitic loads on the engine like alternators, a/c compressors, and power steering pumps. Depending on torque demand and specifics of the engine/transmission, thrashing the battery could also be more efficient than ICE alone.
I'm sure there's stuff I've missed, but I think those are the main reasons why a number of hybrids have highway EPA ratings 10-15 mpg higher than the highest MPG ICE car ever produced.