While You Were Sleeping: February 5th, 2015

The internal combustion engine works best at a high constant speed, and incidentally can be made very clean as to emissions if running that way. Use a small IC engine, running at a high constant speed, to power a generator, which charges batteries, from which an electric motor draws power. In other words, a series hybrid. Using a diesel for this is even more efficient, and the constant speed running allows the emissions issues with diesels to be largely overcome. You can probably power a small car (Toyota Corolla size) with a 40 HP ICE in the series hybrid configuration since the average power draw is lower than that, and during times of peak demand (highway acceleration) the additional power comes from a net discharge of the battery; during lower demand times the engine-generator set provides a net charge of the battery. The Chevy Volt is a half-step toward this, but as far as I know no one in passenger cars has had the nerve yet to do it fully. Instead, for reasons which have always seemed spurious to me, they have released parallel hybrids. Sure, they work well, especially when new, but the penalty in complexity is extreme. I have never seen an actual engineering analysis, with real numbers and data, demonstrating why the series hybrid is a bad idea. I think the series hybrid is a much better idea than these ridiculous n+1 speed automatic transmissions. But you all know better than me, and of course electronics and software never fail.

@turf3 Series hybrid has been tried, but it does not work in reality, and that's why there are few examples (and the BMW i3 is one). The charge/discharge losses are killer. It is better to sacrifice a little bit of prime-mover efficiency (by letting its power output vary to match power demand as closely as possible whenever it can) in order to eliminate the charge/discharge losses whenever it can. Every time you convert energy from one form to another, there is a loss. Starting at the engine flywheel, a conventional powertrain keeps it as mechanical powertrain all the way to the wheels (but is incapable of regenerative braking). A parallel-hybrid also routes most of the power through a gear set (whenever it can) with only a portion going through the electrical system. The various Toyota and Ford hybrids, and to a lesser extent the Volt, are like this. In a series hybrid, there is no choice for that mechanical power to first become alternating current, then direct current, then charge a battery to become chemical energy where it sits for a while, then discharge a battery at a different time to go back to direct current, then be inverted to alternating current, then go into an AC traction motor, and STILL go through a set of reduction gears. There is a loss associated with every one of these conversions. If you keep it all-electric but you allow engine output to vary to match demand whenever it can (steady driving) you can skip the battery charge-discharge loss. But still ... a suitably-chosen set of gears will be more efficient than all of those electric shenanigans under those conditions. When running in combustion-engine mode, neither the Volt, nor the BMW i3, get fantastic mileage.

@turf3 I don't believe that is correct about an IC being most efficient at a high speed. Peak efficiency should be at under a relatively high load at moderate/low speed, probably around the torque peak rpm. Throttle wide open to minimize pumping losses, lowish speed to minimize friction and give time for complete combustion. And a turbo to recover waste energy in the exhaust stream. A great example of this is an early lpt 2.3t Saab 9-5. A 2.3l engine designed to minimize friction, a small turbo to pump up the torque at low rpms, and tall gearing such that a 3500lb luxury 5 pax car could easily get 35mpg on highway trips with late 90's tech. This is also why modern CVTs can be very efficient, and the whole point of many-speed gearboxes. That peak efficiency range is relatively small, you need a lot of gears to stay in it. The ultimate example is a big diesel truck engine with a 1200-1500 rpm usable rev range, and a 10-18spd or more gearbox behind it. Or a bicycle for that matter - humans make peak torque over a narrow range of rpm as well! I think the big problems with series hybrids are both efficiency and performance. To get it efficient enough, the ICE needs to be really small. And then you end up with an i3 where once the battery is run down the "range extender" performance is pretty terrible in comparison. If you make the ICE big enough to have "acceptable" performance, it is a ton of weight to haul around on battery. I still think the range extender on a trailer is the ultimate way to go. Pure electric and leave 500lbs at home when you don't need it, unlimited range when you do need it at the cost of some efficiency.

@turf3 The series-parallel hybrid system seems to be advantageous over just a series hybrid setup. GM found this out designing the Volt, which was supposed to be a series hybrid. The transmission design Toyota came up with for the Prius is absolutely ingenious. It allows for both series and parallel function, as well as creating full CVT transmission. And and all this is in a transmission that's simpler then just about any other automatic. With a pure series hybrid, you still have the motor and generator, so you really don't even get weight savings. In a series-parallel hybrid, the system is always using the combined output most efficiently. If engine output is suited to power the wheels, it does. If it's best suited to charge the battery, it does. If the engine can shut down it does.