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To: carranza2 who wrote (24102)	8/31/2001 9:33:42 PM
From: Maurice Winn	Read Replies (2) \| Respond to of 29986

Carranza, <I wonder if anyone has done a study on the amount of energy which is potentially recoverable from the heat generated by automobile brakes.> having male answer syndrome I can tell you that such research has been done and it is happening right now as I interrogate my keyboard. Let's assume a spherical cow and a 1.8 litre Toyota. In highway mode it gets about 11 km per litre [which isn't litres per 100 km or miles per gallon [imperial]]. When in around-town mode it gets about 9 km per litre due to energy used in accelerating and braking. Driving along a highway at slower speeds than 100 kilometres per hour [meaning city road speeds] would improve efficiency to about 14 km per litre because the air resistance is the square of the speed. Therefore halving the speed saves a LOT of energy in air resistance, but not much in rolling resistance [tyre drag]. So, we have got about 50% of the fuel which could be saved if not for the waste in accelerating and braking. During acceleration, when most of the city fuel is used, the engine is high-revving, inefficient and losing a lot of the energy [about 80% of it goes into exhaust and waste heat in gearboxes, radiators, etc]. So, after an acceleration, only 20% of the energy used to accelerate is still available to be collected in slowing for the next set of lights. But only a portion of that energy is dissipated in braking, depending on whether the driver is a wacko who races up behind all the other cars then climbs on the brakes or looks ahead, eases off ages before the lights and cruises right on through without much, if any braking - though anyone is stuck with a certain amount of braking. Some of it is used in coasting to a stop, using wind and rolling resistance to slow the car [and some engine resistance unless, like me, [in the good old days], you put it in neutral to avoid that waste]. So, perhaps half of that 20% is consumed in brake pads at the next deceleration. We are already not getting much energy back! So, of the extra 50% used in city driving, only a fifth goes into possibly recoverable energy and only half of that goes into the brakes. So, we can recover only about 5% of the total fuel used in a city-only spherical cow by using regenerative braking. Almost nothing is recoverable in highway driving. Of course, that's in a spherical cow. In special city applications, such as a bus, which starts and stops and where braking is a much bigger energy consumer as a proportion of the energy in, it would make sense to have regenerative braking. Especially since the regeneration process can be run in reverse and used as the driving mechanism. The Fisher and Paykel SmartDrive washing machine drive fisherpaykel.com could be used. I will go out to the shed [with 4 of the spare parts from those machines] and make such a car, with a jet engine powered electricity generator, to prove the concept. This washing machine is designed really cunningly. One of the functions is regenerative braking. The motor could be used to drive the wheels on a car, as well as braking them. On a bus, fuel savings of perhaps 30% could be achieved with regenerative braking. That would cut a lot of pollution. I think I might have got those percentages a bit muddled, but you get the idea. Oops, I just remembered, only 70% or so of the regenerative braking energy can be stored due to losses, so the savings are even less. We need horns on the spherical cow to be accurate. Or an udder idea. Another rough way of looking at it is to think how hot brake shoes get around town and think how much heating goes out the exhaust, the radiator, and other cooling. The brakes aren't running all that hot. So there isn't all that much to be saved. 5% for a car seems about right. Maybe. At 2000 litres per year, that's about 100 litres saved x NZ$1 per litre = $200 per year. Also, there would be no brake shoes getting worn out, braking could be controlled much more precisely than ABS brakes, the capital cost of the brakes would be saved [the motors would be needed to drive the wheels so they'd be dual-purpose], no hydraulic fluid so less maintenance and less risk of failure. Hmm, sounds good. I will check the Toyota link and see what dinkum engineers say. prius.toyota.com Mqurice Note: Spherical Cows here uscibooks.com