Heat Pump on Exhaust

[jeff88]

Originally Posted by serialk11r

NO no no. EGR decreases the temperature of combustion because there's less fuel. Since there's less combustible mix, it's actually harder to ignite. Colder spark plugs won't do you any good.

OK, well now I'm confused. Allpar says the exhaust gas does not replace any air or fuel, but you're saying it does?

(Note that the exhaust adds to the fuel and air; it doesn’t replace any of it).

Originally Posted by serialk11r

EGR increases the mass of gas but that doesn't do anything. You need pressure. EGR is only to dilute the intake mix so you need to suck in more of it (which takes less energy because the engine has constant stroke volume and the energy to create the manifold vacuum is by far greater than the energy required to accelerate the air down the intake tracts). If it's not cooled EGR, the charge starts out hotter and you lose efficiency from that, but gain most of it back from the pumping loss reduction.

Doesn't an increase of mass on the top of the piston increase the pressure on the piston? Similar to if you put a mouse on your head vs. putting an elephant on your head, the heavier (massive) one is going to put more pressure on you.

Does hot(ter) EGR decrease efficiency or power? I only wonder, because I'm thinking about how a HAI increases efficiency, but lowers power and (controversially) hot fuel vaporizes easier. Also, how does one tell if their EGR is cooled or not? Is there a seperate 'radiator' of sorts?

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Questions:
1. Theoretically, if one were to lower the combustion temperature (no matter how that happens), would colder spark plugs help, or is the hot/cold spark plugs a different thing?

2. I'm wondering, if increasing EGR, as well as increasing air charge (turbo), could the efficiency be exponentially increased (or at least increased more than one of them separatly)? This would help create a lot of pressure on the piston, (or) open the throttle body more (depending on how you look at it), decreasing vacuum losses.

[Occasionally6]

Whether or not there's less fuel and air depends on what you look at. For a given power output at the crankshaft, the addition of EGR will mean less fuel and air are required, because the pumping loss is less.

If you hold the amount of air and fuel constant, then the EGR is additional and the power output at the crankshaft increases with the lower loss to pumping work.

Some (more) thermodynamics:

Q = m.Cv.(Tf - Ti)

Q is the heat into the combusted gases (~ N2, CO2, H2O) from the fuel, less losses to the cooling system and oil.

m is the mass of the combusted gases, including EGR if applicable.

Cv is the constant volume specific heat.

Tf is the temperature after combustion

Ti is the temperature prior to combustion.

From that you should be able to see that if you add more 'm' and keep 'Q' constant, (Tf - Ti) must decrease to balance the eqn. More mass, same heat = lower temperature.

It is somewhat complicated by the fact that Ti will be higher with EGR both because the EGR is warm and because the pressure is higher prior to the compression stroke with it. Both act to increase the temperature (and pressure) at the top of the compression stroke.

Hot and cold wrt 'plugs refers to how easily they lose heat (to the cooling system, via the cylinder head). A hotter 'plug is easier to fire, reducing the likelihood of misfire, but may act as a glow plug and prematurely light the air and fuel mixture if too hot; pre-ignition.

At the end of the EGR section, it says the reason EGR increases FE (among other things) is because it increases the mass on the piston, helping to force it down. I wonder if the reason why pgfpro's turbo helped increase mileage was because it helped increase the pressure on the piston, forcing it down against the vacuum.

Gravitational force has negligible effect on the force on the piston; mass of charge, not weight.

The pressure on the non-combustion side, 'outside', of the piston is ~atmospheric pressure, not vacuum. Any low pressure - vacuum if you prefer - is always inside the cylinder. EGR does reduce that vacuum. A lean air:fuel mixture would do the same thing but mess with emissions - high NOx.

Cooled EGR does use a heat exchanger to reduce the temp. of the gases.

EGR will reduce pumping loss. Too much will prevent the air and fuel from burning (think CO2 and steam fire extinguisher).

EGR can be used instead of fuel to cool combustion with supercharging.