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Old 12-01-2010, 10:13 PM   #59 (permalink)
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Originally Posted by NeilBlanchard View Post
I have a couple of questions about piston ICE's:

How much pressure could be developed in the combustion chamber *just* from fuel burning? If there was no compression of the intake air (with only a turbocharger in place) would the resulting pressure from the burning fuel be enough to get decent torque? Particularly, if you did not have to "do the work" of compressing the air with the piston -- which uses the momentum of the flywheel, then would the net pressure gain be the same?

Do diesels get all their additional efficiency from the higher compression? Or, are other factors contributing?
Basically the pressure on the piston during the power stroke is the combination of compression and the expanding volume of the heated fuel air mixture. Lets say you have a piston surface area of 10 square inches. Compression ratio is 10 to 1.

Compression pressure will be 147 pounds on the top of the piston. Combustion expands the mixture by about 7 fold. Remember the heat measurement of the air is in Kelvin from the point of the atmosphere being a solid at very low temps to the point of peak pressure at about 3200 degrees.
So your 147 pounds of compression pressure is multiplied by 7 to create 1029 pounds of pressure on the piston surface.

Under the most perfect conditions (not realistic) you must spend the energy to create compression in order to gain the pressure from expansion. Higher compression creates a greater difference in pressures. Higher energy content in the fuel creates greater expansion. Both increase the pressure differential and therefore more useful work is performed.

If you remove (theoretically) all mass and friction from the engine (as a hypothetical situation) then your ideal efficiency would be the difference in the work to compress and the work from expansion. It gets a whole lot more complicated if you consider all factors involved but this is an easier scenario to understand using theoretical absolutes (again not realistic).

Not exactly sure how you are going to make power without compression Neil, but without compression the pressure created from expansion alone would have to be a heck of a lot greater to produce the same amount of work.

Pumping losses, friction, poor mechanical leverage, reciprocating masses, and other factors reduce the efficiency further from the ideal (again not realistic) difference between the energy necessary to compress the mixture and the energy released from combustion.

To delay the peak pressure to a point where the leverage is better (as you referred to previously) the present evolution seems to be towards multiple injections, using direct injection, with a small injection to begin combustion and several more injections to spread out the pressure peak and push the piston with a longer pressure duration. Precision control of multiple injections and high pressure direct injection, allow significantly higher compression ratios without the possibility of detonation even on regular gasoline.

Direct injection at pressures many times higher than peak combustion chamber pressures from fuel ignition make this possible. Mazda has just come out with a gasoline engine that has 14 to 1 compression, using the above mentioned multiple injections per combustion event.

As far as supercharging and running leaner mixtures than normal. I will require something like the Transonic type of injectors at this point in time, but rest assured there are many R&D efforts in process with the goal of true HCCI where the mixture is as close to perfectly distributed during combustion. If this can be accomplished outside of a laboratory then the byproducts of combustion will be reduced to the point where after treatment may not be necessary.

As far as supercharging i like the idea of electric supercharging (just my opinion) because it can be configured to be available when necessary without any continuous operation of either a supercharger or turbocharger. This will allow downsizing of the engine to increase BSFC during normal low load operation.

Idle elimination and any operation outside of best BSFC, as demonstrated in the INNAS design can be accomplished by capacitive storage and release of energy, but it requires an IVT of some configuration to apply power from a reserve that is never in the same state of energy storage. In other words to apply energy at a steady power level from a diminishing storage state, you have to be able to constantly adjust the rate of release.

Such a system could also be used in an electric vehicle to be a "load leveler" for stabilizing battery discharge rates avoiding high discharge rates and allowing recharging to occur at lower sustained rates over longer periods of time.

The manufacturers will always try (as long as they can get away with it) to avoid any more than incremental steps in improvement in their designs, because they don't want to make last years models obsolete as well as prior years to a certain extent, until about 7 years into the life cycle of the cars in service.

Although I advocate elimination of reciprocation altogether (already discussed) there are methods to minimize the penalty of energy losses due to reciprocation by incorporating better design and materials in pistons, and connecting rods. Titanium is one example, but probably not yet cost effective. Investment casting of intricately designed geometric configurations to reduce the weight of reciprocating components could make a difference.

Variable compression is another way to improve efficiency, but it will only be effective if you have less that peak BSFC loads applied on the engine which should be avoided in the first place.

Throttle control should also be eliminated, and in some designs coming to market now this has been done. Honda has gone in a different direction with their ISDI engine reverting back to a 2 valve configuration with dual spark plugs. They claim they can reach efficiency levels of the old lean burn Civic engine without the NOX issues, but that is older tech compared to the newer direct injection combustion strategies.

I think there is a future in 2 cycle engines, but the stigma of the old designs is a hard prejudice to overcome. The cost of compression could be reduced by injecting the fuel and air into the engine in a homogenized state with no valves and exhaust ports at BDC to allow combustion by products to escape with a significant amount of residual byproducts for the purpose of EGR.

Some of these suggestions are ways to make what exists better, while others will require a complete reconfiguration of the commonly known IC engine. When the new configurations have been worked out and perfected, I think you will find there is a lot of life left in the IC engine, at least until the cost of liquid fuels become exorbitant. Much will depend on the future of many different technological pathways, a better battery, better electrolysis, better synthetic fuels, better fuel cells, whichever becomes practical reliable and cost effective will shift the pathway of future development.

Personally I thin you will see vans of the size of the Dodge Sprinter (Mercedes) achieving 40-50 MPG averages within the next 10-15 years.

These are all my observations and opinions. If you have a better one I would like to read it as a post with some links to back up the post.

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