[peppercorn (OP)]

I mean this:


with this:



More clearly a free (crankless) piston Otto cycle with two pistons firing alternatively.
The exhaust gas to run a flash steam boiler used to return pistons for next compression/exhaust stroke.

The double free piston cycles like this:


with the steam piston (not shown) returning them to top-dead-centre.

[LeeE]

In gas turbine engines, water injection doesn't directly boost power.  Instead, it cools the engine, allowing more fuel to be burnt without damaging the engine through overheating.  The power boost comes from being able to burn more fuel.

I thought it was the same principle in operation with water-injected I.C. engines, especially when combined with methanol injection.  I'm less sure about this though.

[Geezer]

Peppercorn,

Not quite what you are proposing, but I seem to remember that Sulzer developed a power generation prototype where a number of free piston diesel engines did nothing but produced hot gas to drive a turbine. All the work was derived from the turbine. The diesels were really only "combustors".

If I can dig up the link, I'll add it on.

[peppercorn (OP)]

...you can inject the water directly into the gas engine as shown below.  The heat of the gas burning in the cylinder vaporizes the water droplets in the cylinder...

I've come across quite a number of articles on water injection with two main theories as to how water causes a power boost. One is as you say the expansion of the water to steam heavily increases displacement causing more work (and subsequently a lower exhaust temp).
The other theory claims a lot more importance on the atomised water at the intake dropping the charge temperature prior to compression - whereby a cooler charge can pack more air into the cylinder (as LeeE's post implies).
Both make some sense, but the problem is that the water droplets can coalesce with the fuel to create a poor burn (which is arguably where the methanol comes in).

The other problem is now to fluid have to be 're-fuelled' i.e. petrol & water. Also the exhaust gas would be unsuited to catalytic converters (except, on the plus side, NOx wouldn't be likely to form at lower ignition temps).

Hey, that sounds pretty complicated and, without a crank, what uses does it have?

Yes, it's complicated, but so are hybrids & extra expansion piston systems, etc.
Also flash boilers are a low-tech device with low manufacturing overheads: Remember, I only propose to get enough force to get the pistons moving up again (work normally done by the flywheel & inertia in rotational engine).

The free piston engine can be utilised for driving wheels by either a hydrostatic drive or the motive element in a pneumatic hybrid.

[peppercorn (OP)]

Peppercorn,

Not quite what you are proposing, but I seem to remember that Sulzer developed a power generation prototype where a number of free piston diesel engines did nothing but produced hot gas to drive a turbine. All the work was derived from the turbine. The diesels were really only "combustors".

If I can dig up the link, I'll add it on.

Thanks. I'd like to read that! I did consider something similar, where the work from the Diesel IC was just used to compress a larger amount of air to replace the compressor-stage on a GT.

[Geezer]

Here's the Sulzer link. You might get more information from the patent. I seem to remember there was one.

http://en.wikipedia.org/wiki/Diesel_engine#Gas_generator

[peppercorn (OP)]

Here's the Sulzer link. You might get more information from the patent. I seem to remember there was one.

http://en.wikipedia.org/wiki/Diesel_engine#Gas_generator

I found this:


Don't know what advantages it offers over a standard GT, though.

[LeeE]

There are some aspects of that drawing that don't quite make sense to me.  It might just be that some of the labels don't point to the right things, or that other important aspects of the design have been omitted, but something I definitely find a bit weird is that the drawing appears to show an axial flow turbine.  The gas plenum would have to be huge (and with a correspondingly long charge time) to stand any chance of delivering the sort of smooth flow that would really be needed to drive an axial flow turbine with any degree of efficiency.

[Geezer]

The Sulzer system used multiple free-piston diesel engines driving a single turbine, which would produce a more constant gas flow.

[LeeE]

Unless the combustion in a diesel engine is considerably more efficient than the combustion in a conventional constant gas generator turbine i.e. a jet turbine, I just can't see how this type of engine could be more efficient, incorporating not only its dual stages, but also the transmission line between them.

[Geezer]

Unless the combustion in a diesel engine is considerably more efficient than the combustion in a conventional constant gas generator turbine i.e. a jet turbine, I just can't see how this type of engine could be more efficient, incorporating not only its dual stages, but also the transmission line between them.

I would not disagree with you. It's just an interesting alternative method that might have benefits in certain situations.

[LeeE]

Oh, it's interesting, I'll grant you that, but it's interesting in the same way that Giant Pandas are interesting []

[Geezer]

Oh, it's interesting, I'll grant you that, but it's interesting in the same way that Giant Pandas are interesting []

Is it possible to attain the same sort of compression ratios and combustion temperatures that a diesel achieves? If not, that might have a real impact on efficiency. Also, a diesel has a very immediate heat recovery mechanism in that the next air charge takes up some of the waste heat from the previous combustion cycle. Also, it may be less expensive to build because the diesels replace the compressor section and combustors, and further, the diesels can "spin-up" more quickly than the fan compressor. In some situations that could be important, on the other hand, if it was such a good idea, why is it not an available product?

It may also have a flatter power/speed relationship than a conventional GT, but if it is used to drive a generator, I don't think that is too important.

[peppercorn (OP)]

Also, a diesel has a very immediate heat recovery mechanism in that the next air charge takes up some of the waste heat from the previous combustion cycle.

Not sure if the science is right for heat recycling argument.
Correct me if I'm wrong, but wouldn't cold air entering a hot cylinder before the inlet valve closes start expanding, so reducing the charge and lowering efficiency?

[Geezer]

Also, a diesel has a very immediate heat recovery mechanism in that the next air charge takes up some of the waste heat from the previous combustion cycle.

Not sure if the science is right for heat recycling argument.
Correct me if I'm wrong, but wouldn't cold air entering a hot cylinder before the inlet valve closes start expanding, so reducing the charge and lowering efficiency?

Are you concerned that the expansion will prevent sufficient air from entering the cylinder? I don't think it's a problem as long as a sufficient mass of air is admitted to achieve complete combustion.

[peppercorn (OP)]

Are you concerned that the expansion will prevent sufficient air from entering the cylinder? I don't think it's a problem as long as a sufficient mass of air is admitted to achieve complete combustion.

Yes. It does appear as if expansion before detonation would be working against the engine's inertia. Also, by heating the air prior to ignition less expansion work is possible - so the power is less (and, taking the Carnot heat engine model - So is the efficiency [I think]).

Cold air inlet systems & forced-charging both push up the power (although not, I'll admit, necessarily the efficiency) by increasing the charge.

[Geezer]

I believe it can only improve efficiency. GTs preheat intake air from the exhaust, and some even use mechanical recuperators for this.

In a diesel (or petrol engine), the heat applied to the air charge reduces the waste heat that will be dissipated into the atmosphere via the radiator. Also, the increase in air temperature results in a higher combustion temperature for a given amount of fuel. That improves efficiency. Of course, combustion temperatures must be limited to prevent damage.

[peppercorn (OP)]

I believe it can only improve efficiency. GTs preheat intake air from the exhaust, and some even use mechanical recuperators for this.

As I understand it, the exhaust heat recuperation takes place only after the compression stage has ended - as the warmer the air is prior to entering the compressor the less compression work is possible.

In a diesel (or petrol engine), the heat applied to the air charge reduces the waste heat that will be dissipated into the atmosphere via the radiator.

How? If you are talking about exhaust heat then this should have no effect on the (block) cooling system. AFAIK, the only time intake is purposefully warmed is when the engine (or weather) is cold - to encourage good fuel atomisation and burn.

Also, the increase in air temperature results in a higher combustion temperature for a given amount of fuel. That improves efficiency. Of course, combustion temperatures must be limited to prevent damage.

I was under the impression that is isn't, per se, the final exhaust temp that is important. Rather it's the ratio of the temperatures before and after ignition. The ideal situation for piston engines is starting with the coldest air for a fully compressed charge (that will still result in complete combustion). Then, immediately after, the highest peak temperature in the centre of the air charge (that won't crack the conn rod, etc or melt the piston/cylinder).

[Geezer]

I believe it can only improve efficiency. GTs preheat intake air from the exhaust, and some even use mechanical recuperators for this.

As I understand it, the exhaust heat recuperation takes place only after the compression stage has ended - as the warmer the air is prior to entering the compressor the less compression work is possible.

Warm air is less dense, so the air mass is reduced. However, all that matters is that you have enough air to combust the fuel that you need to burn to produce the deisred amount of power.

In a diesel (or petrol engine), the heat applied to the air charge reduces the waste heat that will be dissipated into the atmosphere via the radiator.

How? If you are talking about exhaust heat then this should have no effect on the (block) cooling system. AFAIK, the only time intake is purposefully warmed is when the engine (or weather) is cold - to encourage good fuel atomisation and burn.

No. Not exhaust heat. Heat in the combustion chamber. The intake of air is an important aspect of 4-strokes because it helps to prevent the combustion chambers from overheating.

Also, the increase in air temperature results in a higher combustion temperature for a given amount of fuel. That improves efficiency. Of course, combustion temperatures must be limited to prevent damage.

I was under the impression that is isn't, per se, the final exhaust temp that is important. Rather it's the ratio of the temperatures before and after ignition. The ideal situation for piston engines is starting with the coldest air for a fully compressed charge (that will still result in complete combustion). Then, immediately after, the highest peak temperature in the centre of the air charge (that won't crack the conn rod, etc or melt the piston/cylinder).

In a typical IC engine, about one third of the energy from the fuel goes out the exhaust. Another third is dissipated as heat from the radiator. The only way to improve the efficiency significantly is to prevent, or recover energy from those sources.

Higher combustion temperatures do improve thermal efficiency, but not all that much.

[peppercorn (OP)]

Warm air is less dense, so the air mass is reduced. However, all that matters is that you have enough air to combust the fuel that you need to burn to produce the desired amount of power.

Having enough air just to combust all the fuel is not all that matters, otherwise no one would bother with lean-burn technology.  A piston engine still needs a working fluid i.e. the part that isn't oxygen - it just happens that the reactant for the fuel (O2) and the working fluid (mainly nitrogen) are contained in one handy gas - Air.
The amount of expansion the working gas can go through from being heated is reduced the closer it is to the combustion temp of the fuel & O2. Additionally, more heat will reach the cylinder walls if the working fluid is hotter - i.e. the energy has to go somewhere.

No. Not exhaust heat. Heat in the combustion chamber. The intake of air is an important aspect of 4-strokes because it helps to prevent the combustion chambers from overheating.

So the cooler the air the better for the cylinder walls.

Higher combustion temperatures do improve thermal efficiency, but not all that much.

No, but assuming all other aspects are equal it is not worth ignoring.
Note: I said greater difference in pre & post combustion temperatures NOT just the combustion temperature alone.