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Discussion Starter #1
Been thinking about this one for a while now. Once plug-in hybrids come on line, engine torque and driveability will be much less significant, as the electric buffer will be so big the only thing that will be wanted from the engine will be as high outright thermal efficiency as possible.

At the moment the best petrol engine efficiency you can get is in the Prius - about 35% under ideal conditions. However, VW's best TDi engines are now at about 46% efficient. This means that drop for drop, the TDi engine can allow a vehicle to travel 31% further (ie it is able to extract 31% more energy out of every drop of fuel than a spark ignition engine).

As I see it, most of this improvement is down to lack of throttle, and the diesel engine's high compression ratio (20-24:1). Surely, therefore, it would be trivial to make a petrol fuelled compression ignition engine? I read once that old Peugeot diesels were meant to take up to 25% petrol in their diesel tanks during the winter - and on the farm I have seen in the past diesel engines run (accidentally) on about 50% petrol with no ill effects at all (other than much easier starting).

Is there some reason why spark ignition must be used with petrol? Certainly stochiometry must be maintained in these engines (ie amount of air and fuel must match), but this is not the case with diesel engines. In fact, petrol engine efficiency plummets when not used at full throttle, and this loss isn't nearly as pronounced in diesel engines.

Final question - what are peoples opinions on the feasibility of an Atkinson (or Prius-like Miller) cycle diesel engine? Would likely not be able to rev more than about 3,000rpm, but a potential expansion ratio of 30:1 could mean an efficiency maybe in the 60%s?

:)
 

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Considering that engine response time need not be critical in a hybrid with a sufficiently large electric motor and battery, I'd like to see a car that used purely electric to drive the car, and a Stirling cycle engine to charge the battery. Such a car would have far greater flexibility for choice of fuels, could be cleaner with less technology, and with an adequately-sized battery could have great flexibility for "stealth" mode.
 

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clett said:
As I see it, most of this improvement is down to lack of throttle, and the diesel engine's high compression ratio (20-24:1).
I think the VW TDi is about 19:1, but since it is turbocharged (up to about 1.2 bar) the total exhaust expansion is more than just the geometric 19:1
Surely, therefore, it would be trivial to make a petrol fuelled compression ignition engine? I read once that old Peugeot diesels were meant to take up to 25% petrol in their diesel tanks during the winter - and on the farm I have seen in the past diesel engines run (accidentally) on about 50% petrol with no ill effects at all (other than much easier starting).
I think that petrol is only used to prevent gelling of diesel fuel in cold weather.

Is there some reason why spark ignition must be used with petrol?
Not sure if petrol would ignite as readily well as diesel when sprayed into hot air. Aren't there some direct injection petrol engines coming out? I suspect they still use a spark.
Final question - what are peoples opinions on the feasibility of an Atkinson (or Prius-like Miller) cycle diesel engine? Would likely not be able to rev more than about 3,000rpm, but a potential expansion ratio of 30:1 could mean an efficiency maybe in the 60%s?
Never heard that discussed before but it seems feasible if a turbo wasn't used. It seems to me the Miller is a cheap, slightly mickey-mouse (and therefore short term) way for Toyota to get a little more efficiency since the ECVT can readily compensate for the loss in flexibility. They need to watch costs to help offset the price of the hybrid system parts. A turbo and related parts would add too much cost even though it is probably a more efficient way to get more expansion.

A better way of optimizing efficiency (with a "simple" petrol engine) might be a variable geometry cylinder such that combustion chamber size (not throttling) can be used to vary output power. Peak compression pressure would be constant at all power levels for a given fuel octane.

Isn't it amazing that after 100 yrs of development we are just scratching the surface in improving engine efficiency?

My supervisor in college developed a commercial Stirling cycle generator. Might be a good possibility although in any case I think Toyota are on the right track by passing most of the mechanical power directly to the wheels.
 

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Paul said:
clett said:
Is there some reason why spark ignition must be used with petrol?
Not sure if petrol would ignite as readily well as diesel when sprayed into hot air. Aren't there some direct injection petrol engines coming out? I suspect they still use a spark.
Mitsubishi and VW/Audi already have GDI (gas direct injection) engines on the market, and there could be more. Audi wins the LeMans series each year with this engine because of the performance/fuel efficiency characteristics. They haven't been able to pass US emission standards with this engine yet. I heard that we will see GDI from Audi in the US within the next year or two.
 

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It's interesting watching gasoline & diesel engines develop. Gasoline engines are borrowing ideas from diesel (direct-injection/no spark plug) to improve fuel economy, and Diesel engines are borrowing from gasoline engines (exhaust gas recirc, turbo, lower compression) to lower emissions. They are slowly but surely moving towards one another.


Do a search. Lots of people are working on compression ignition engines for gasoline. One problem they are having is the trottle & how to control engine speed without it.
 

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ElectricTroy said:
Do a search. Lots of people are working on compression ignition engines for gasoline. One problem they are having is the trottle & how to control engine speed without it.
That's interesting. MG1 (the motor/generator on the sun gear) in the Prius has enough of a mechanical advantage over the engine that it could be used to control engine speed. I suspect that it actually does in the Prius, but haven't figured out how to prove it.
 

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I guess you need to provide more information about the problem you think needs to be solved.

You originally said engine speed needed to be controlled without a throttle. I suggested an electric motor/generator with mechanical advantage. You said that removed energy from the system in a non-productive way. I suggested not producing so much energy. Now you say a motor is not a good solution with no further illumination of the problem. I'm stuck without more hints.

The spambots are an unfortuate annoyance. The primary forums on this board are moderated pretty quickly, so you seldom have to see the spam on them. These less used forums aren't moderated as often, so you have a higher chance of seeing it here. But it does get cleaned out eventually. Note that the one between your message two posts ago and my previous post is now deleted.
 

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Seems to me the spam could be eliminated by not allowing "guest" posters.



Clarification:
- A compression ignition engine that runs on petrol/gasoline will rev to 10,000 and blow up. That's because there's no air throttle.

- you suggested this:
electric motor------> <-----------engine
to retard the engine, but that would be energy wasteful, so that's not a good solution.

- You then suggested control the fuel injection. That works for diesels, but for some reason, it's doesn't work for gasoline. (I'm not an engine designer, so I don't know why.)

That's the obstacle researches are trying to overcome:
- how to stop the engine from racing to 10,000 rpm
- but the diesel technique of "fuel throttle" doesn't work
 

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Well, obviously I'm not going to solve a problem here that researchers can't solve with real equipment to work on.

But electric motor speed control along with some means of limiting energy output of the engine still sounds like it might work. Maybe they could run the engine for a few seconds with the motor drawing off power to a battery (or ultracapacitor?), then use the battery to move the car for a few minutes while the engine is off. Think of it as PWM for engines :) Or, don't open the intake valves every cycle. Maybe one in 10 for light power requirements, one in three for medium, and every cycle to climb Pike's Peak (using the weight of the car to control engine speed). Use electromagnetic lifters instead of cams.

Of course, if it turns out the real problem is that they can't retain enough heat in the cylinder for compression ignition to work, glow rings might need to make a comeback.

If it gets too complex to be cost effective, Daniel's Stirling cycle engine - electric motor hybrid may beat them to it.
 

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Discussion Starter #13
ElectricTroy said:
- A compression ignition engine that runs on petrol/gasoline will rev to 10,000 and blow up. That's because there's no air throttle...

... control the fuel injection. That works for diesels, but for some reason, it's doesn't work for gasoline. (I'm not an engine designer, so I don't know why.)
Any idea why it won't work for gasoline too? In a diesel engine, all you do to control engine speed is inject less fuel at every cycle. If that didn't work in a gasoline CI engine, and it spun up to high rpms without any fuel being injected then surely that's a perpetual motion machine! BTW, BMW's valvetronic (gasoline) engines use the amount the inlet valves open to control how much air is taken in and make do without a big air stirring throttle - improves efficiency a fair bit.
 
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The main issues with a petrol compression engine would likely be these:

1. The reason that diesel actually get's more miles to the gallon, is because there is more thermal energy in a gallon of diesel. If you switch to gas, you lose that advantage.

2. Oxides of nitrogen. Horrible smog causing pollutants which are a by-product of the diesel combustion cycle. This occurs in diesel much more than in gas engines because diesel combustion takes place at higher temperatures. The nitrogen in the atmosphere will combine with oxygen under these conditions to make a prime ingredient in smog.

Although this measure of air quality is largely ignored in Europe, American air quality standards have been specifically geared against smog since the '70s. Those who live in LA or Denver understand why.

Nitrogen oxide as a component of smog is produced completely independent particulate componentry the exhaust. The latter can be scrubbed out to some degree, but the former requires advanced urea injection which is only now being introduced in Europe.

...but I ramble. If you have a petrol compression engine, you'll likely have the same problem with oxides of nitrogen.
 

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Discussion Starter #16
Yes, you're right that diesel does contain more energy per gallon than gasoline (up to 18% more, depending on where you are / time of year etc). But you still can't escape the inherent efficiency advantage of CI over spark ignition (SI). If the typical peak efficiency of a gasoline engine is 30% (Prius, with Miller cycle, is ~36%), the best road-going diesels are still at around 45%. To my mind this would mean that if a gasoline powered car was getting 30mpg with SI, it could be getting 45mpg if the engine ignition mechanism was switched from SI to CI (ie 50% further on the same amount of fuel).

Of course the nitrogen oxides thing is still a big outstanding issue to be solved, but it's not one that's insoluble (IMHO) and urea injection is not the only solution being looked at. This (and the particulates issue) are the main reasons why diesels are so unpopular in Japan, but the major manufacturers there are now realising the benefits of CI and are making great progress into solving these (Toyota in particular).

Nevertheless, I still think the major reason gasoline engines are SI and not CI is because people just don't like the sound of the "diesel-clatter" you get on older CI engine designs. The best thing we can do for these people is to just take them for a spin in a new Accord diesel or BMW 330D!
 

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Our Insights & Civics have high NOx output (the lean-burn), but Honda simply used a catalyst to neutralize it. The same can be done with diesel.

Compression Ignition IS more efficient. If you compare a Beetle Spark engine vs. Beetle Compression engine, the efficiency translates to ~2000 BTU/mile vs. ~3000 BTU/mile.
 
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