The future of the engine as we know it 1

[franzh]

I normally run like crazy when someone asks me questions about a "new" engine, but when one of my directors asks me to evaluate a "new design" design, my knees shake. That happened this morning.

We have seen free pistons, opposed oscillating pistons, scotch yoke pistons, toroidal blocks, ball valves, sliding sleeves, and tons of other combinations. Strangely, or not so strangely, none of these has ever reached any significant success. The old, inefficient Otto cycle engine still hangs around. With the exception of sealing refinements and air-fuel metering, almost nothing has changed in 100+ years, not including OHC engines which is a refinement, or phased cams, also a refinement, or exhaust aftertreatments, which is not a refinement, but an attempt at fixing an engine inefficiency.

I wonder how long it will take before something else pops up and takes a reasonably strong stand against the tried and true engine design?

Lets look at the true IC engine replacement, not hybrids (although I feel that is where we are heading for the next 25 years). Turbines are nice but not practical for a number of reasons.

When we look at H2 as a potential fuel, it still uses the Otto cycle engine as a base, only the fuel has changed. Are we really gaining anything on the engineering side? I recognize the significant advantage on emissions, but fuel transport, storage, and infrastructure have still to be perfected.

Any ideas, fellow engineers?
Franz


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[Warpspeed]

Two stroke or four stroke is only really relevant where there is only a single working cylinder.

With multiple cylinders you can have as many firing strokes per revolution as you wish with either.

 

[j2bprometheus]

Warpspeed-

Adding more cylinders adds more cost.

The proper comparison is single cylinder to single cylinder or eight cylinder to eight cylinder.

j2bprometheus

 

[globi5]

BMW's steam assisted drive:

http://www.greencarcongress.com/2005/12/bmw_developing_.html

One thought: If the steam engine was charging batteries or condensators via a generator, the waste heat could be used to supercharge the engine with an electric supercharger, therefore allowing a much smaller, lighter engine with better fuel efficiency at low power settings (which could possibly offset the extra weight of the steam engine with generator).

Of course and as always the question will be: Are the added costs justified?

 

[dalcazar]

What happens to a flywheel when you hit a wall going 65 mph? I can see it breaking off its shaft and flying out of the car or breaking up into shrapnel.

I've given thought to the use of compound turbocharging and the use of a stirling engine powering an alternator to charge batteries.

The compound turbocharger doesn't really seem to work because as I understand it you need to compress the air going into the engine to be able to extract the power from the exhaust, or else the turbine at the outlet produces backpressure in the engine (I could be greatly mistaken and tell me if I am because I'd like to know more). So therefore you can only really generate power by using the blowby relief valve when you're finished exerting the engine and you have too much boost, this is fine for a mining truck or a stop-and-go vehicle, but not much use on the highways at sustained speeds and throttle settings because your extra boost goes away.

Alternatively, you can use a stirling engine to convert the waste heat from the exhaust and the block to power a generator to charge batteries or drive a flywheel (read the first part of my post as far as worries for the second option go), the advantage of which is that the engine is always generating excess heat, and once the batteries are full you just divert the electric power to supplement the IC engine's power by driving electric motors. the disadvantage is the added weight of IC engine + stirling engine + electrical motors.

Diesel hybrid seems to be the way to go in the near future tho.

another interesting engine can be seen here:

http://www.microturbine.com

they aren't a miracle engine or anything else, and being turbines they have fuel consumption problems. the big advantage they have is that they only have one moving part, but there is no contact between moving parts whatsoever so they simply don't wear out, the only maintenance required is an air filter change every so many hours. they're already being used to power buses. You end up with a vehicle that could run for many hundreds of thousands of miles without need for a tuneup or oil change. The use of charging stations at home would also help the hybrids.

 

[globi5]

Scania has been selling a turbocompound system on its trucks for a while now.

http://www.scania.com/products/trucks/Technology/Scania_turbocompound/how_does_it_work.asp

(If it wouldn't work they wouldn't sell it.)

Not sure whether this has even been questioned, but BMW's turbosteamer (as opposed to the turbocompound system) doesn't increase backpressure.

I wonder whether BMW's mini steam turbine has a higher power to weight ratio than a stirling engine. (There must be a reason why BMW's engineers didn't come up with a stirling engine.)

 

[dalcazar]

Interesting, I wonder if the flywheel can redirect it's energy back to the compounding turbine, that would allow it to maintain it's rpms and thus avoid backpressure, and if so the engine can spin up the compounding turbine when starting up so that avoids it as well. But, this isn't something you can do with an electric turbocompound unless you want to use up the energy you already put into the batteries to spin it back up. thanks for pointing that out

as for stirling engines, the best gas you can use is hydrogen, and there are big problems with keeping it inside the engine since it'll leak out of anything. I can see a steam turbine being easier to maintain over time.

I had heard about this following link before, CAT and john deer have been working on an electric turbocompounder.

I stumbled across this link while looking for some up to date info on the topic so you might find it very interesting, the pdf document is contained in there but I'll provide the link to that one as well.

http://www.osti.gov/fcvt/deer2005/deer2005wkshp.html

the specific link is here

http://www.osti.gov/fcvt/deer2005/Vuk.pdf

very interesting stuff!

 

[GregLocock]

Power to mass ratio for stirling engines

http://www.cirps.it/old/attuali/interna_19.htm

3.5 kW 200kg (complete installation including burner), say 17 W/kg

However, NASA reckons 15 W/kg max, admittedly for space qualified gear.

Rather interestingly none of the Stirling engine sites I found that claim improved power to weight ratios actually quote numbers, so I think they are shuffling their feet, whistling, with their fingers crossed, hoping I'll go away.









Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[j2bprometheus]

Hi-

The brake thermal efficiency of the stirling engine that GregLocock found is terrible ---> less than 10 % ----- must have a poor regenerator.

Admittedly, they are using the rest of the heat to make hot water. But the electricity is more valuable than hot water.


j2bprometheus

 

[globi5]

Any additional turbine will increase backpressure. As does a muffler, just without doing any useful work.

 

[Warpspeed]

Yes, but a flash boiler could extract a lot of otherwise wasted exhaust heat with very little added back pressure, and muffle the exhaust at the same time. The steam supply would go up and down with engine power with very little lag, I suspect few if any additional control systems would be required to make it work.

Generating high pressure steam is an absolutely brilliant idea. Anyone know what sort of steam expander they use ? From the rather poor quality picture in the press release, it looks like two internally geared rotors of some type. A screw expansion engine perhaps ? What a wonderful idea.

 

[dalcazar]

A company called STM Power (

http://www.stmpower.com)

has a 55 kW stirling module that weighs 4000 pounds (as stated in the newsweek article linked to their page) so you get around 30 W/kg. A tenfold increase in power output. These units are apparently for sale already and have been shipped to customers.

Even then a unit weighing 100 kg only makes 3 kW or nearly 4 HP, nothing compared to the steam thingie. I like that idea.

 

[SMOKEY44211]

In keeping with the original theme of this thread I want to restate that I think the IC piston, rod, crankshaft engine will be the prime mover for suface transportation for the forseable future. Given the amount of energy that is lost to the exhaust and cooling system provides a source of potential recovery. Turbocompounding seems to be the most logical choice for exhaust energy recovery. The Wright Cyclone TC3350 is well documented showing about 20% improvement in BSFC #s and power increase. I think the Scania effort is compleatly backwards. Turbine driving the crank should be ahead of the turbo supercharger. I suspect the arrangement they offer was the most cost effective method of implementing it on their existing powerplant arrangement. The data claims that they are getting about 5% higher BSFC #s and power at first glance seems a little on the low side but there is less energy lost in the exhaust with a diesel vs a gasoline fueled engine to begin with. I also think that generating steam from the heat lost in the cooling system has a lot of merit. Steam spins turbine linked to the crank or final drive line and use the radiator as a condenser. I only became aware of the BMW effort a few days ago via the link provided by GLOBI5. It doesn't appear that they are giving any data about fuel economy improvements or power levels. I think there is a 20%-30% (gasoline fueled engine) improvement available there. Not sure at this time how to address the ice problem in sub freezing climates. Implementing one or both of these methods should keep the IC engine around long enough to power the hearse that delivers me to my final resting place.--------Phil

 

[SMOKEY44211]

Forgive me I over looked the 15% fuel economy gain from steam stated from the BMW link.-------Phil

 

[globi5]

At least as far as I know the Wankel engine has hotter exhaust gases than a conventional gasoline engine. The Mazda RX-8 actually has an electric air pump forcing air into the exhaust for post combustion:

http://www.rotaryengineillustrated.com/renesis/renesis16popup.jpg

)
So the steam assisted drive in conjunction with a rotary engine should be more effective than with a conventional engine. Maybe this could help make Wankel engines more popular - especially since the Wankel engine is supposed to have a higher mechanical efficiency (no valve train)?

 

[globi5]

'found a picture of a stock RX-8 with an aftermarket exhaust:

http://intinsifi.com/transmit/flames.jpg

(It at least appears to lose some heat. Note: there's no crazy exhaust fuel injection or something).

 

[turbocohen]

Wankel thermal efficiency is quite low due to the higher surface to volume of the combustion chamber. The Wankels advantage is power to weight and packaging size. A derivative of the Wankel makes a decent supercharger though.

 

[globi5]

The higher surface to volume ratio is one reason why the thermal efficiency of the Wankel is lower - the other is incomplete combustion due to the moving and slim combustion chamber (that's why the air pump is necessary).
However some claim that at high loads a Wankel is more efficient than a conventional gasoline engine. I read somewhere that one of the reasons why Mazda's 26B won LeMans 1991 was because its rotary engine was more efficient than the turbocharged conventional engines of the competition (fuel was limited). Maybe someone has more info about this?

If the steam engine would drive a supercharger, the surface to volume ratio would go down and the power to weight ratio of the whole system would be increased or at least be unchanged.
This axialflow supercharger only weighs 11lbs.

http://www.axialflow.com/products.html

 

[dalcazar]

As far as I know the main reason why the mazda 26B won the LeMans race was because the only thing it ever needed to stop for was gas and tires. The truth is, wankel engines get horrible gas mileage, even the RX-8 with it's supposed improvements gets worse gas mileage than a similarly powered car with a normal reciprocating engine. (the typical comparison is with the nissan 350Z)

by the end of the race, the engine in the 26B had virtually no wear, whereas a normal engine would probably be in moderate to poor shape since they're made to last one race only before a rebuild.

 

[globi5]

The 26B won because its reliability as well. But I remember reading somewhere that it had a low fuel consumption compared to other race cars.
Here's a link:

http://www.monito.com/wankel/lemans.html

Couldn't find the fuel consumption in g/(kW*h) though.

A passenger car is driven 95% at partial throttle and a race car 5% at partial throttle. If we are comparing fuel consumption of passenger vehicles we really compare fuel consumption of these cars at partial throttle.

Unfortunately there isn't a lot of fuel consumption data of race cars available.

 

[dalcazar]

well with the link you gave, the 787B gets 4.31 miles per gallon. doing 213 km/hr average

I've estimated the fuel consumption of the audi R8 at around 5.8 miles per gallon under wet conditions with a pit stop after 200 miles (exact mileage unspecified) and an 80 gallon tank of gas. this data is all I've been able to find for any other car other than the 787. the track was wet for at least part of the time on this run so it's slower than normal, but then again the fuel consumption is a very conservative estimate. Also, the R8 won the best fuel consumption award at the 2005 lemans race (which it won) so it's not a fair comparison.

any racers know a '91 fuel consumption figure?