I Know Nothing About Car Systems

[wdb]

Each time a cylinder's compressed charge is ignited a violent force is applied to the top of the piston, through the connecting rod, and into the crankshaft. The flywheel absorbs some of that violence and stores the energy in the form of motion (inertia). As some have noted, a single cylinder 4-stroke engine might be totally incapable of running without a flywheel because it could not keep rotating long enough to get back to the power stroke.

I would say that most other engines could run without a flywheel, but the engine - and everything attached to it - would experience considerable vibration. The more cylinders, the less vibration. But adding cylinders adds complexity and, importantly, friction, so there is a practical limit to the number of cylinders an engine can have.

The other end of the spectrum is the "hit-and-miss" engine, in which a single cylinder fires only occasionally and a ginormous flywheel keeps things moving in the interim. Cool beasties, and you can probably see one at your local "heritage days" festival.

http://en.wikipedia.org/wiki/Hit-and-miss_engine

[some_other_dave]

Originally Posted by Occasionally6

It might be interesting to investigate what combination of ignition and exhaust valve timing defines the limits before there would be a possibility of the firing cylinder trying to turn the engine backwards.

I remember hearing a couple of stories many years back about a scooter with a 1-cylinder 2-stroke engine. I think there were occasions when it would backfire (significant pre-ignition) for some reason, and some of those times it would actually start running backwards.

-soD

[Occasionally6]

Originally Posted by Occasionally6

The pressure in the cylinder at the bottom of the expansion stroke is always higher than that at the top of the compression stroke in the next cylinder to fire.

I need to check this. If it is not so, 4 cylinders won't be enough, at least not without the crankshaft acting as the flywheel.

Originally Posted by jeff88

Could pressure equalization, intake/exhaust timing and spark advance/retarding make it possible to eliminate the flywheel or is something else involved?

There is still going to be a minimum number of cylinders required. It would not be possible to run a single cylinder engine without some inertia in the crankshaft to provide the work for the 3 non-firing strokes.

[Occasionally6]

I did a quick check using the assumption that the cycle is adiabatic, air only as working fluid. Yep, pressure at end of expansion stroke is much greater than that at the end of the compression stroke.

[wdb]

Originally Posted by Occasionally6

It might be interesting to investigate what combination of ignition and exhaust valve timing defines the limits before there would be a possibility of the firing cylinder trying to turn the engine backwards.

I assume you're talking about 4-stroke engines, but 2-stroke engines are pretty easy to get running in the 'wrong' direction. A buddy of mine kick-started a 2-stroke Saab backwards once upon a time, and ended up with 4 reverse speeds and 1 forward!

[jeff88]

This isn't so much car related as it is a necessary evil for all cooling needs (homes, refrigerators, cars, etc.). I was researching how an A/C works for another one of my ideas and then it hit me. Why does the coolant need to be compressed? Why can't it just go from heat sink (evaporator) to cooler (condenser) and continue to work within the loop?

[mechman600]

I'll take this one.

Refrigerators and A/C systems work essentially the same way. Refrigerants are weird fluids that are a gas at room temperature and atmospheric pressure but change into a liquid when compressed - 70 PSI (or so) at room temperature.

The fundamental function of the refrigerant is the change from a liquid to a gas. A liquid contains much more heat energy than a gas, and changing a liquid into a gas requires a lot of heat energy.

What happens is your refrigerator pump (or A/C pump) compresses refrigerant into a liquid, pumps it through the condenser (back of the fridge or in front of your radiator) to get this liquid fairly close to ambient temperature. It then goes through a tiny orifice at the entry to the evaporator. What this orifice does is reduce the pressure to below a point that the refrigerant can remain a liquid. The refrigerant then becomes very very cold because it has changed from a liquid to a gas. Changing it into a gas requires a lot of heat energy, which is absorbed from its surroundings. We see it as being very very cold.

Ever open a propane bottle and see how stinking cold the liquid coming out is? Same exact thing.

Your heater fan blows air through the super cold evaporator to cool your cabin. The refrigerant then goes back to compressor where it is compressed back into the condensor. However, it now contains much more heat energy (absorbed in the evaporator) so now that it is compressed back into a liquid, it enters the condensor very hot (too hot to touch) where it is cooled back down. And on to the evaporator again. ETC.

It's all about heat transfer - from the cabin airflow through the evaporator INTO the refrigerant and OUT OF the refrigerant into the airflow through condensor.

[Occasionally6]

I will pick up on that the refrigerant is still a gas as it comes out of the compressor. The compression raises the temperature of the gas, relative to ambient. When the gas is subsequently cooled - heat transferred out of it to, usually, the surrounding air - while still under high pressure, then it will condense into a liquid.

In order for heat transfer to occur, there must be a temperature difference. The compression and expansion are required to generate that temperature difference. If you just tried to pump a cooling fluid - like water - around there'd be little or no temperature difference and therefore no heat transfer occurring.

You can use something other than the common refrigerants in your system. We've discussed using air, which is free and there's no issue with leakage. The problem is that when there's no change of state i.e. condensation or evaporation, only sensible heat energy is available. (Sensible heat involves a change in temperature; latent heat involves a change in state i.e. evaporation/condensation, or freezing/melting.)

The amount of sensible heat per kg of any substance is much lower than the latent heat per kg. As one example, the specific, sensible heat of water is 4.187 kJ/kg.K and the latent heat of evaporation, water to steam, is 2287kJ/kg. That means it takes ~5.4 x more heat energy to boil water than it does to raise the temperature from 0C to 100C. That is why air isn't usually used, unless it's convenient to do so or there's some other reason why a refrigerant is less attractive eg. aircraft, where weight is also important.

The state of a particular substance depends on both temperature and pressure. You can boil water at a lower temperature than 100C if you reduce the pressure over it. (It still takes ~ the same 2287kJ/kg though.)

You could operate your refrigeration or A/C system using water if you were to keep it under low enough pressure.

[jeff88]

OK, so the big thing is getting it to a liquid state. The change from liquid to gas is what causes the 'cold' (like a propane tank). It would be nice if the half million webpages devoted to 'how an a/c works' would say that instead of just detailing the refrigerant going from device to device in a loop.

I was trying to find a pressure vs. temperature curve for R134a, but I couldn't find one. Do you know what temperature it has to be at while maintaining atmospheric pressure to turn into a liquid (i.e. without compressing it)?

I'm wondering if there is a substance that will compress easier than R134a (for efficiency purposes) or if that's the best option.

O6, did you mean that you can boil water at less than 100C if you raise the pressure? Hence the reason why Denver has a harder time boiling water and a pressure cooker has an easier time.

[Occasionally6]

Originally Posted by jeff88

OK, so the big thing is getting it to a liquid state. The change from liquid to gas is what causes the 'cold' (like a propane tank). It would be nice if the half million webpages devoted to 'how an a/c works' would say that instead of just detailing the refrigerant going from device to device in a loop.

It is the compression and expansion which is important. The change of state just makes it more effective and efficient.

I was trying to find a pressure vs. temperature curve for R134a, but I couldn't find one. Do you know what temperature it has to be at while maintaining atmospheric pressure to turn into a liquid (i.e. without compressing it)?

Thermodynamic Property Table for saturated R-134a (pressure table), SI units

Refrigerant R134a Properties

http://www2.dupont.com/Refrigerants/...o_prop_eng.pdf

(-15F or -26C)

I'm wondering if there is a substance that will compress easier than R134a (for efficiency purposes) or if that's the best option.

Probably not, at least not without other issues. Propane would work (is used sometimes - not anywhere near where I am though ) if you could be 100% certain that it couldn't leak. NH3 (ammonia gas) works but is corrosive and a leak is unpleasant. CO2 works OK and looks to be (one of) the gases used to replace R134a in cars.

O6, did you mean that you can boil water at less than 100C if you raise the pressure? Hence the reason why Denver has a harder time boiling water and a pressure cooker has an easier time.

No, definitely reduce. You can boil water when at altitude at a lower temp. The issue is when you're trying to cook something in that water the temp. can be too low and it takes longer to cook things.

It works the other way too. Car cooling systems are kept under pressure to prevent the coolant from boiling.