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Priuses manage to get 50 mpg highway without depleting charge or plugging in but I guess the same optimizations can't work on a big diesel as on a little gas en

Priuses manage to get 50 mpg highway without depleting charge or plugging in but I guess the same optimizations can't work on a big diesel as on a little gas engine?

borner791 3y ago
Prius is just a little gas engine in a little and aerodynamic car. They can have a tiny gas engine because the electric helps acceleration from a standstill. The batteries are just weight on the highway
- adfiogn1i2o 3y ago
  The Prius is not especially small. The engine is 2.0 liters, which is not small at all. It is much more sophisticated than you let on.
- jojobas 3y ago
  >The batteries are just weight on the highway 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 3y ago
    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.
    - jacquesm 3y ago
      > Diesels are most thermodynamically efficient at low throttle I think you meant 'low RPM'. Low throttle doesn't always equate low RPM and VV.
      - thsksbd 3y ago
        See [1] for reference. 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 [1] https://archive.org/details/in.ernet.dli.2015.134070/page/n183/mode/1up?view=theater https://archive.org/details/in.ernet.dli.2015.134070/page/n1...
        jacquesm 3y ago
        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.
- enragedcacti 3y ago
  > The batteries are just weight on the highway 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 3y ago
    "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." 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.
    - enragedcacti 3y ago
      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.
thebigjc 3y ago
https://www.edisonmotors.ca/ https://www.edisonmotors.ca/ is making diesel electric trucks. They only claim a 5-10% improvement on long haul loads.
MadnessASAP 3y ago
Prius's are city cars, city driving has widely varying loads which are inherently awful for combustion engine efficiencies. The hybrid drivetrain allows the engine to stay within its optimal efficiency range which has a huge impact on mileage. Compare this too semis which spend most of their life on the highway with drivetrains optimized almost exclusively for constant highway speeds. There is far less gains to be made converting to a hybrid system. A Prius without a hybrid drivetrain and with an engine optimized for 25 mph driven constantly at 25 mph would actually exceed the standard Prius's mileage since it doesn't have to carry batteries, generators or electric motors.
- thsksbd 3y ago
  This. The genius of the prius is that it manages to iron out the fundamental problem of energy conversion in transportation - no constant cruise speed
theodric 3y ago
Priuses are slippery little kammback hatches with hard tires and a drivetrain designed to minimize Cd and stop-start traffic fuel consumption. A semi is a brick that bullies the wind out of the way all day every day for two million miles before needing a rebuild. They're very different animals for very different use cases.
- thsksbd 3y ago
  Its a long brick though, it shouldn't be considered a bluff body
  - yossarian88 3y ago
    Sure, the wake is thinner relative to the length, but that's not the relevant variable. The drag is still way higher for the same frontal area than for the same width body that's shaped like a prius. and thus power required is much higher.
saalweachter 3y ago
The relevant number for a semi isn't "miles per gallon" but "gallons per (cargo)ton-mile". (Which is not to say you couldn't make some improvements, but you're never going to get 50 mpg while hauling 40 tons.)
to11mtm 3y ago
There's a few things going on that come to mind. Big trucks often have other systems that historically can be dependent on having a good consistent power draw source. Also, air brake systems; The brakes on a power unit are designed to 'fail safe', so rather than using hydraulic fluid to close the calipers, uses air pressure to keep the calipers open. There's a bunch of efficiency implications here but in general you either have to make a work around to that or take efficiency losses of a PTO or electric compressor. Also, not sure whether a different approach requires DOT approval or not. 'What about all those wheels and regen braking' one might ask? Then you'd be talking replacing/retrofitting trailers which has a bunch of implications for an owner/operator. Oh. Transmission losses, and the realities of a diesel; One thing that really helps the Prius (and vehicles that use similar arch) is that it (and many/most of the more efficient hybrids) use a Fixed-Ratio transmission; The engine uses various techniques to maximize efficiency at a wide band. Diesels, on the other hand, are often most efficient within a (relatively) much more narrow power band... Also in general, rig diesel engines aren't exactly designed for 'stop-start' patterns. Then, you've got the delta-V problem; The more weight you add the more energy it takes to move stuff. My SWAG (i.e. completely uninformed and friday night math) is that a power unit would have to have a battery with the capacity of a plug-in hybrid to provide meaningful value. Then, drag coefficient. A truck has a much larger profile from every angle which has a sizable impact on efficiency. Oh, and make it all with expectations for long service intervals.

Priuses manage to get 50 mpg highway without depleting charge or plugging in but I guess the same optimizations can't work on a big diesel as on a little gas engine?

borner791 3y ago

Prius is just a little gas engine in a little and aerodynamic car. They can have a tiny gas engine because the electric helps acceleration from a standstill. The batteries are just weight on the highway

adfiogn1i2o 3y ago
The Prius is not especially small. The engine is 2.0 liters, which is not small at all. It is much more sophisticated than you let on.
jojobas 3y ago
>The batteries are just weight on the highway 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 3y ago
  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.
  - jacquesm 3y ago
    > Diesels are most thermodynamically efficient at low throttle I think you meant 'low RPM'. Low throttle doesn't always equate low RPM and VV.
    - thsksbd 3y ago
      See [1] for reference. 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 [1] https://archive.org/details/in.ernet.dli.2015.134070/page/n183/mode/1up?view=theater https://archive.org/details/in.ernet.dli.2015.134070/page/n1...
      - jacquesm 3y ago
        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.
enragedcacti 3y ago
> The batteries are just weight on the highway 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 3y ago
  "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." 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.
  - enragedcacti 3y ago
    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.

thebigjc 3y ago

https://www.edisonmotors.ca/ https://www.edisonmotors.ca/ is making diesel electric trucks. They only claim a 5-10% improvement on long haul loads.

MadnessASAP 3y ago

Prius's are city cars, city driving has widely varying loads which are inherently awful for combustion engine efficiencies. The hybrid drivetrain allows the engine to stay within its optimal efficiency range which has a huge impact on mileage. Compare this too semis which spend most of their life on the highway with drivetrains optimized almost exclusively for constant highway speeds. There is far less gains to be made converting to a hybrid system. A Prius without a hybrid drivetrain and with an engine optimized for 25 mph driven constantly at 25 mph would actually exceed the standard Prius's mileage since it doesn't have to carry batteries, generators or electric motors.

thsksbd 3y ago
This. The genius of the prius is that it manages to iron out the fundamental problem of energy conversion in transportation - no constant cruise speed

theodric 3y ago

Priuses are slippery little kammback hatches with hard tires and a drivetrain designed to minimize Cd and stop-start traffic fuel consumption. A semi is a brick that bullies the wind out of the way all day every day for two million miles before needing a rebuild. They're very different animals for very different use cases.

thsksbd 3y ago
Its a long brick though, it shouldn't be considered a bluff body
- yossarian88 3y ago
  Sure, the wake is thinner relative to the length, but that's not the relevant variable. The drag is still way higher for the same frontal area than for the same width body that's shaped like a prius. and thus power required is much higher.

saalweachter 3y ago

The relevant number for a semi isn't "miles per gallon" but "gallons per (cargo)ton-mile". (Which is not to say you couldn't make some improvements, but you're never going to get 50 mpg while hauling 40 tons.)

to11mtm 3y ago

There's a few things going on that come to mind. Big trucks often have other systems that historically can be dependent on having a good consistent power draw source. Also, air brake systems; The brakes on a power unit are designed to 'fail safe', so rather than using hydraulic fluid to close the calipers, uses air pressure to keep the calipers open. There's a bunch of efficiency implications here but in general you either have to make a work around to that or take efficiency losses of a PTO or electric compressor. Also, not sure whether a different approach requires DOT approval or not. 'What about all those wheels and regen braking' one might ask? Then you'd be talking replacing/retrofitting trailers which has a bunch of implications for an owner/operator. Oh. Transmission losses, and the realities of a diesel; One thing that really helps the Prius (and vehicles that use similar arch) is that it (and many/most of the more efficient hybrids) use a Fixed-Ratio transmission; The engine uses various techniques to maximize efficiency at a wide band. Diesels, on the other hand, are often most efficient within a (relatively) much more narrow power band... Also in general, rig diesel engines aren't exactly designed for 'stop-start' patterns. Then, you've got the delta-V problem; The more weight you add the more energy it takes to move stuff. My SWAG (i.e. completely uninformed and friday night math) is that a power unit would have to have a battery with the capacity of a plug-in hybrid to provide meaningful value. Then, drag coefficient. A truck has a much larger profile from every angle which has a sizable impact on efficiency. Oh, and make it all with expectations for long service intervals.