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Old 12-06-2010, 08:18 AM   #147 (permalink)
t vago
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Originally Posted by NeilBlanchard View Post
The peak pressure is bending the crankshaft sideways, rather than spinning it. It spends a relatively long time doing this. The connecting rod position relative to the crank center is one of the main reasons that current gasoline engines are so horribly innefficient -- 20-30 sucks, frankly.

And the pressure disputes as the volume expands. So by the time it has the maximum mechanical torque at 90 degrees past TDC, the pressure has dissipated a lot. In a steam engine, the pressure stays virtually constant all the way through the stroke.
There's a reason why we don't use constant pressure processes to extract work. Say you have a cylinder arrangement of some starting volume v1, and some ending volume v2 which is some multiple of v1. (incidentally, that's called the compression ratio). Work is extracted from this cylinder arrangement, and can be calculated.

Now, we have to model two cases here. The first case is of this so-called inefficient process of allowing a maximum pressure at v1, and allowing that pressure to fall as v1 expands to become v2. The second process is the other process of allowing pressure to remain constant while v1 expands to become v2, as in the steam piston.

The first process is called an adiabatic, or constant heat energy process (since no heat energy is being added or subtracted here). Remember that the first process assumed a maximum pressure at v1. For a real gasoline engine where pressure builds up as the combustion inside the cylinder proceeds to completion, and as a little heat energy actually leaves in the form of waste heat into the cylinder walls, this is an approximation, but is fine for our purposes of illustration.

The second process is called an isobaric, or constant pressure process. Note that heat energy is continually added in the form of more steam, in order to keep the pressure constant inside the cylinder. Again, there will be a little heat lost to the cylinder walls, but this can also be ignored for now.

The reason I ignore the real-world heat energy loss and internal combustion pressure increase behavior is that it can be assumed that the process happens quickly enough so that heat energy loss can be assumed to be zero. Similarly, the combustion and resultant pressure increase can be assumed to be instantaneous, since we are talking about a time span of milliseconds here.

Given that, we can set up a couple of thermodynamic equations and solve for the amount of useful work that will be extracted in either case. I will not bore you with the equations in this particular post, since a lot of boredom already was generated in the above paragraphs (but will be more than happy to post the equations here, if asked).

It turns out that you can extract some quantity of useful work w, given the adiabatic process. Now, and this is the important part here - you can extract about 0.23 w from the isobaric process. That's less than 1/4 of the work that was extracted from the adiabatic process.

Even if you divide the work output of the adiabatic process by 2, to simulate the 4-stroke nature of an internal combustion engine, you end up having a process that is still more than twice as efficient at extracting useful work than a steam engine cylinder.

Originally Posted by NeilBlanchard View Post
The Revetec engine nearly doubles the efficiency of the bog standard design. So does the Atkinson cycle Prius engine.
This is misleading. The Revetec engine is still a dream, as is this other Scuderi engine I looked at last night. You would think that it would be possible for them to have provided something by now, if this engine were as efficient as is claimed. Scuderi has had 9 years to develop their engine. Revetec has had even longer (15 years). Surely, if nothing else, a small engine based on either design could have been made available to the general public by now, if for nothing more than for powering lawn mowers or similar.

As for the Prius engine? It has to do a job that is different from a normal IC engine. You've noticed that the engine is more efficient? You will also notice that power and torque are reduced in the Prius engine, compared to an otherwise equivalent gasoline engine, and that's so that the Prius engine can more effectively drive an electric propulsion system. Also, the Prius engine has no parasitic loads it has to drive (unlike a standard gasoline engine).

In order to make a gasoline engine more efficient, you're going to have to sacrifice some power and torque somewhere. That's the nature of the beast.

Originally Posted by NeilBlanchard View Post
The valve train is another major drain on efficiency -- it takes a lot to push open the typical poppet valves that require springs. Though, I don't know if there is an efficiency gain with a desmodromic valve train like Ducati's?
The beauty of springs are that you get that energy back when the springs expand. The real losses in a valvetrain are due to frictional losses, not springs. Besides, you're talking about a loss of perhaps 1 HP, if that. Reduce the friction losses in a valvetrain by 25%, and you'd be lucky to see a 1% increase in fuel economy.

Originally Posted by NeilBlanchard View Post
I am trying to get people to think unconventionally -- what we are doing can only be tweaked; when we need a quantum leap. Rotary engines (not necessarily Wankels) have an response to the crankshaft issue -- some of them at the very beginning of this thread look very promising.

My proposed design was an attempt to answer these problems without abandoning the crankshaft.
You are going to run up against engineering realities, even with thinking unconventionally. It's nice to have a pretty idea for a new-wave engine process, but it will have to run against the real world with all of its limitations. Materials engineering, and pesky real-world behavior that refuses to agree perfectly with ideal conditions, result in what you see today.
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