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dcb · 01-18-2010, 01:30 PM

I've herd a few things lately that need more discussion. Namely with series hybrids like the volt and aptera appearing and folks assuming that running the engine at bsfc peak regardless of other "driveline" losses is the most efficient way to move a car. I don't believe it is, given a reasonable gearbox and a good driver, and a will and acceptable conditions (a personal decision) to drive for efficiency.

The thing is that the gas engine is billed as a "range extender", and to me that means usually a hiway trip for business/pleasure. I think a gas only solution would use less gas than a hybrid for this extended range mode, where an electric vehicle can handle the local "i.e. 40 mile radius" usage where there is a higher percentage of braking.

I've never seen anyone do any efficiency analysis on a series hybrid, as far as I can tell there are many losses to consider:

engine to generator
generator to controller
controller to motor
generator to charger
charger to battery
battery to motor
transmission/wheels

whereas a small gasser can use
engine to transmission/wheels

now that is a large number of energy conversions going on for the series hybrid, it is off to a real bad start. Lets also remember that these conversions are also not all at their peak efficiency all the time, motors and generators do not have a flat rpm/current/efficiency curve, nor do batteries have a flat efficiency curve.

So what of the benefits of bsfc peak? Well lets look at an example chart for a reasonably efficient engine:

Look at the small center "target" oval, @250. You can drive this car at within ~90% of bsfc peak from 1300 rpm to 4300 rpm!!! that is a huge rpm range. And the only equipment you need is a brain and a right foot, or from 40% load to %75 load if you have the rpm dialed in (i.e. coping w/hills). Of course this is best leveraged by pulsing and gliding around bsfc, and you do have other gears to climb hills with near bsfc peak and just glide down them.

So will all those conversions of the series hybrid outperform a "john henry" driver in terms of efficiency? I doubt it will, at least not before gas becomes obsolete anyway.

I do like the parallel hybrid short term, especially if it is just a small motor & wheel you stick in the hitch receiver of your electric vehicle, that is just sized to push you down the hiway near bsfc peak (the EV takes up the slack). To me that is pretty close to perfect (and similiar things have been done), as you still only buy one car and can go anywhere with it with minimal fuss, but move your daily commute to electric operation without waiting for the holy grail of batteries.

01-18-2010, 01:30 PM	#1 (permalink)
dcb needs more cowbell Join Date: Feb 2008 Location: ÿ Posts: 5,038 pimp mobile - '81 suzuki gs 250 t 90 day: 96.29 mpg (US) schnitzel - '01 Volkswagen Golf TDI 90 day: 53.56 mpg (US) Thanks: 158 Thanked 269 Times in 212 Posts	The problem with series hybrids I've herd a few things lately that need more discussion. Namely with series hybrids like the volt and aptera appearing and folks assuming that running the engine at bsfc peak regardless of other "driveline" losses is the most efficient way to move a car. I don't believe it is, given a reasonable gearbox and a good driver, and a will and acceptable conditions (a personal decision) to drive for efficiency. The thing is that the gas engine is billed as a "range extender", and to me that means usually a hiway trip for business/pleasure. I think a gas only solution would use less gas than a hybrid for this extended range mode, where an electric vehicle can handle the local "i.e. 40 mile radius" usage where there is a higher percentage of braking. I've never seen anyone do any efficiency analysis on a series hybrid, as far as I can tell there are many losses to consider: engine to generator generator to controller controller to motor generator to charger charger to battery battery to motor transmission/wheels whereas a small gasser can use engine to transmission/wheels now that is a large number of energy conversions going on for the series hybrid, it is off to a real bad start. Lets also remember that these conversions are also not all at their peak efficiency all the time, motors and generators do not have a flat rpm/current/efficiency curve, nor do batteries have a flat efficiency curve. So what of the benefits of bsfc peak? Well lets look at an example chart for a reasonably efficient engine: Look at the small center "target" oval, @250. You can drive this car at within ~90% of bsfc peak from 1300 rpm to 4300 rpm!!! that is a huge rpm range. And the only equipment you need is a brain and a right foot, or from 40% load to %75 load if you have the rpm dialed in (i.e. coping w/hills). Of course this is best leveraged by pulsing and gliding around bsfc, and you do have other gears to climb hills with near bsfc peak and just glide down them. So will all those conversions of the series hybrid outperform a "john henry" driver in terms of efficiency? I doubt it will, at least not before gas becomes obsolete anyway. I do like the parallel hybrid short term, especially if it is just a small motor & wheel you stick in the hitch receiver of your electric vehicle, that is just sized to push you down the hiway near bsfc peak (the EV takes up the slack). To me that is pretty close to perfect (and similiar things have been done), as you still only buy one car and can go anywhere with it with minimal fuss, but move your daily commute to electric operation without waiting for the holy grail of batteries. __________________ WINDMILLS DO NOT WORK THAT WAY!!!