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.
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.
enragedcacti|2 years ago
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.
thsksbd|2 years ago
I agree with everything you said, except for this. Is this really true? The Geo Metro is rated 53 mpg highway. The 80s VW diesels had good mpg too.
adfiogn1i2o|2 years ago
It is much more sophisticated than you let on.
jojobas|2 years ago
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.
thsksbd|2 years ago
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.