Re: 'Steam' powered cars...

[Ian Stirling]

> In Autocar this week.

>> >> BMW are experimenting with using the waste heat from an IC engine to >> produce steam which drives a motor attached to the main engine. On the >> test bed they're getting a 15% saving in fuel consumption - at the >> same time as more power and torque. >> >> Seems it's not a new idea, but BMW reckon it is practical and should >> be in production within 10 years. > > When I was a nipper my old man was a tinkerer in electric motors and > generators and although this is very vague he put together on a piece of > wood a small cycle light generator a 12 or 24 volt motor(?) and rechargable > battery connected together whereby the battery started the motor which in > turn rotated the generator wheel thus fed power to the battery for charging > purposes, obviously there was some electronics in between which as i said > was very vague recollection

not. However, the BMW gadget suffers from none of this - car engines are quite inefficient (thermodynamically), and the exhaust temperature (at the exhaust valve) is around 700C.

Current car engines are about 25% at best efficient, so 75% of power goes out the exhaust pipe wasted. Theoretically, about 66% of this could be recovered by a completely efficient heat engine, but in practice, even large power stations only get about 50%, and they start off a bit hotter, as well as having ready access to cooling water.

10% is probably achievable - in the long term, for maybe a recovery of 7.5%, or a gain in efficiency of 30%. This means however major additional systems such as much, much bigger radiators.

[Ian_m]

Best cars, maybe 18% efficiency, power stations lucky if 25%, fluidised coal bed stations can reach 33% efficiency.

[Newshound]

Can't remember the formulae following the Xmas lunch, but I think modern coal/oil plant is knocking on 40% and CCGT more like 50

[Chris Bacon]

I thought most was lost through the radiator.

[Paul Hubbard]

If the head gasket goes on any car, you could sa it is steam powered..! :o)

[Edward W. Thompson]

What class of power station? Coal fired steam turbine with/without passout, gas turbine, gas turbine with cogen, Low speed heavy fuel diesel engine with waste heat recovery,? There are many combinations the most efficient of which is likely the slow speed heavy oil diesel engine with waste heat recovery that will exceed 50% efficiency, however, this type of generation is not, I believe, used in the UK. Industrial gas turbine with cogen is probably a very close second, again with 50% plus efficiency.

[Peter Hill]

Cars are about 30% efficient at peak, usual operation is about 25%. Cooling power = engine power is reasonable rule of thumb and assume the remainder (33% up to 50%) goes down the exhaust pipe. Some of the cooling is direct to air from hot under bonnet surfaces. Not enough heat goes down exhaust pipe to run ancillaries at town road speeds and idle. 70mph only needs about 35bhp, so maybe as little as 35bhp or up to 50bhp goes to exhaust. They claim to extract 15% extra giving about 5bhp at 70mph which would just about do for the ancillaries but not with air-con.

[news07]

In message , Edward W. Thompson writes

Baglan Bay in South Wales is the worlds most efficient (non-cogen) power station with an expected test efficiency of around 60% (Google GE 9H gas turbine), add in cogeneration and you can expect the thermal efficiency to exceed 70%.

Your typical coal fired station like Drax or Didcot will be around 36% efficient.

[Andy Dingley]

Didcot isn't quite typical - it has long been used as a technology demonstrator for the latest in efficient turbine technologies.

[Andrew Sinclair]

Erm I worked at Didcot A (the coal fired one) for several years and the turbines there weren't anything special, run of the mill Parsons built AEI design. Over the fence at the gas fired B station it was the launch site for the Siemens V94.3a gas turbine which was pretty advanced in the mid-1990's but technology has moved on. Typically we refer to gas turbines in classes, the machines at Didcot B are F class, the GE machine at Baglan Bay is H class (there are G class machines out there but not many, mainly in the USA and Japan).

Rugeley is just about the most efficient coal fired station in the UK, used to regularly win awards when the industry was nationalised.

[The Natural Philosopher]

Nope. Exhaust.

[Andy Dingley]

So what about the variable stators? Parsons were quite proud of them when they built them (retro-fit, not original). I worked on a vibration sensing project for them in the late '80s, detecting flutter in the blades.

AIUI, the intention behind all this was to improve efficiency at startup, thus allowing the plant to be brought on and off load more quickly, without so much of the usual inefficiencies.

[Johannes]

Strangely enough, cooling though the radiator is just as important as the heating in the combustion for achieving high efficiency. You can only extract mechanical energy between two temperatures; the larger the gap, the more efficient.

[PC Paul]

So what does the radiator cool that forms the 'cold' part of that pairing?

[mrcheerful]

The driver's head?

mrcheerful

[PC Paul]

I suspect that may be the case here.

While the theory is basically sound, last time I looked my car didn't have a Stirling engine under the bonnet...

Which recent cars do have chargecoolers, or even intake air coolers which are cooled by the rad? I know none of mine have...

[marb]

Near enough all turbo charged cars including diesels!

[Dave Plowman (News)]

That's a bit of an over simplification. Engines don't run at their most efficient until up to temperature. Probably to do with proper atomisation of the fuel.

[Andy Dingley]

Name one _car_ (as opposed to a boat) that uses a water-air intercooler. Intercoolers are reasonably common (although by no means "near enough all") but they're air-air intercoolers, not water-air.

If you fit a water-air intercooler to a car, you'd just have to enlarge the radiator to dump the heat out that way. Space is tight under a bonnet, so they use air-air instead.

The question here was radiator vs. exhaust, not radiator vs. exhaust vs. separate intercooler.

[Johannes]

Indeed, this is abstract thermodynamic theory that was fully developed in the 19'th century. It is unrelated to a particular engine as long as it's in the business of converting heat to mechanical energy. The maximum possible efficiency of an engine is (T1-T2)/T1 .