How to get 120HP from a 76HP engine
OK, There have been some fair comments made, and the concept behind the hybrid transmission is difficult to grasp. I seem to have unwittingly become an unofficial spokesman for the hybrid "religion", so I'll try to explain the technology. I assume that Register readers are technically literate and of above average intelligence, so I'll now explain how to bend the laws of physics without actually breaking them.
The first thing to get straight is that the Hybrid-vs-Diesel argument is a red herring or, as we say over here, a complete Furphy (look up the origin of that phrase of you want). The hybrid concept is a type of TRANSMISSION, not a type of ENGINE. It just happens to be simpler to attach it to a petrol engine than to a Diesel engine, for various technical reasons which I won't go into. Its main feature is that it makes it possible to have a peak power much greater than the available engine power. Thus most hybrid cars have much better performance than their raw engine power would seem to suggest. Over here in Australia the 1.5 litre Prius is reckoned to have about the same performance as a typical 2.5-3.0 litre Aussie car.
Also, in the Toyota system, it acts as a very efficient automatic transmission which has an infinitely variable gear ratio with very few moving parts. The Toyota "Hybrid Synergy" system is very clever (I wish I'd thought of it), but very simple mechanically. It just consists of a simple epicyclic gear mechanism (basically a type of differential) coupled to two electric motor/generators. The clever part is in the electronics. By controlling the power feed between the two motor/generators and the battery, any gear ratio can be selected from full forward through neutral to reverse, all without any mechanical gear changing. Also, over most of the speed range, most of the engine power is going through the highly efficient gear mechanism, rather than through the less efficient generator -> motor route. Unfortunately, Toyota protected the essentials of the idea very effectively with a series of patents, which is why none of the other manufacturers can use it (although I believe GM is negotiating a licence deal with them)
To explain the concept more thoroughly, I'll now quote from a blog entry I wrote in one of the Australian newspapers. This particular quotation was made in response to a suggestion that hybrid cars were "cheating" by storing vast amounts of energy in the battery where it couldn't easily be measured. Here goes: First the quotation and then my reply
>> Given that the energy stored in the battery can only come from energy from the fuel burnt in the engine (or braking) it would only be fair to compare a Prius' fuel economy with other cars, starting with an EMPTY battery. I suspect that this does not occur so the figures we see are 'artificial'.>>
No Paul. You've misunderstood the hybrid concept. The energy stored in the battery is tiny - a few spoonfuls of petrol at most. Its purpose is to provide a buffer between the energy output of the engine and the (constantly varying) energy demand made by the driver. This allow the engine to always work at or near its most efficient point.
Let me explain. The efficiency of a petrol engine varies enormously, depending on speed, throttle setting, temperature and several other factors. At its best it's reasonably good - around 30%, or a bit higher for the Prius, which runs an "Atkinson" cycle. This is achieved with a wide-open throttle at low revs - in other words, when it's labouring. At its worst, when idling in traffic, its efficiency is zero - it's burning petrol and going nowhere. Unfortunately, a labouring engine gives a rotten driving experience and we spend far too much time in traffic jams.
The trouble with a normal car is that the engine is the only source of motive power, so it has to be sized to provide the maximum power you'll ever need, even though you may only need that much power for a few seconds every day. Most of the time you're using less than 10% of that power, which means the engine is working in a very inefficient part of its operating range. Also, the engine speed is tied to the road speed, through the gearbox. Even the best gearbox can't keep the engine at its most efficient speed over much of that range.
The hybrid concept is to size the engine according to the average power demand (plus a decent safety margin), and then use the battery plus electric motor to smooth out the peaks.
The fact is that, in most cars, you can't apply peak power for more than about 10-15 seconds, or you'd be breaking the speed limit. The battery only needs to cope with that short-term peak, plus a safety margin to cope with unusual conditions such as steep hills.
The result is that the petrol engine can be made much smaller and lighter. Moreover, since it's controlled by the on-board computer, it's always run close to its most efficient point (It's not directly connected to the wheels). For example, when going downhill or decelerating, it is often stopped altogether (Also, the surplus power is collected and put back in the battery, rather than heating up the brakes). Similarly, when crawling through traffic jams, it's just run for a few seconds every few minutes to keep the battery topped up. By the way, the heating and air-con are driven from an on-board three-phase power bus, just like in a building. All this is done automatically. You don't need to think about it - just drive it.
The overall effect is that, although all the power comes ultimately from the petrol engine, the engine is always running at or near the point where it gives the best efficiency and least exhaust emissions. In contrast, a normal car engine spends most of its time a long way away from that point.
