ignition timing vs. engine temp

[Chris H.]

Could someone please explain how ignition timing effects coolant temperature. Thanks, Chris

[Greg]

Re: ignition timing vs. engine temp

As I understand it, with the ingition retarded, less energy is expended in pushing the pistons and more is expended in exhausted heat.

Any other points of view?

Greg

[Duke W.]

Re: ignition timing vs. engine temp

If we look at the chemical energy contained in the fuel about one third is turned into useful mechanical energy, one third is dissipated in the cooling system and, one third goes out the exhaust. These are ROM approximations for an engine operating at WOT at the torque peak. When an engine is running on the dyno we can set the ignition lead for peak torque and this will also usually yield the best brake specific fuel consumption and the coolest EGT. If we retard the timing from this value, torque will go down, and EGT and bsfc will go up, so we have altered the balance of the fuels energy - less is turned into useful mechanical energy and more goes out the exhaust.

In pre-emission days, timing was generally advanced as much as possible without detonation with a combination of initial, centrifugal, and vacuum advance. With early emission control is was necessary to increase EGT to promote the oxidation of HC and CO is the manifolds and upper exhaust system, so initial timing was cut back, centrifugal advance was often slowed down, and vacuum advance was not activated at idle or often in the lower gears (TCS). This helped achieve emission standards, but resulted in higher fuel consumption and less torque in the normal driving range where emissions are measured during certification testing.

Because the exhaust ports are surrounded by the water jacket, a higher EGT transfers more heat to the coolant, and thus increases coolant temperature (once the thermostat is nearly full open) for a given size radiator and fan performance.

[Michael H.]

Re: ignition timing vs. engine temp

That's a very informative dissertation, Duke. I'm having a little trouble with the acronyms, though: EGT, bsfc, ROM, and TCS?. I could only figure WOT is wide open throttle.

Thanks! -Michael

[Duke W.]

Definitions

EGT - exhaust gas temperature

ROM - rough order of magnitude

TCS - transmission control spark, an emission control system that disables vacuum advance in the lower gears, common on late sixties to late seventies GM cars including Corvettes.

bsfc is brake specific fuel consumption. A well designed spark ignition engine will usually be around 0.5 pounds per horsepower-hour at the power peak. Or, in other words, the engine consumes 0.5 pounds of fuel for every horsepower developed over a one hour time period. This is usually attained with a mixture slightly richer than stoichiometric, which is the theoretically "correct" fuel/air ratio that would result in all fuel and oxygen consumed if combustion was perfect. Richer than stoichiometric yields more power because a stoichiometric ratio always leaves a bit of oxygen unreacted, so a bit more fuel will make more power at the expense of lower bsfc.

[Dale Pearman]

STO ITCHY TORQUE

Well said Duke, and if we are reading from the same book, peak torque occurs at maximum volumetric efficiency, with a slightly higher than stoichiometric ratio.I agree that BSFC suffers but the valves stay a tad cooler, which is desirable.

Another useful definition: RITFLMAO...Rolling in the floor laughing my ass off

Reverend Varooom!

[Terry M.]

Re: ROTFLMAO *TL*

Dale,

Must be you Tennessee folks - we certainly need the spell checker, and then we need John to program it with all these fancy engineer words - up here in the north we roll ON the floor. Sometimes on other parts of the building too, but that is another story.

Stociowhat?? It must be the weekend. Think that one is in the spell checker?

Get your LAP TOP out or is it up?

Terry

[Dale Pearman]

Re: ROTFLMAO

You need to see the kinds of floors here in Tennessee. Beleive me, we roll IN the floor.

Rev. Varooom!

[Bill Braun #33186]

Re: ROTFLMAO

Rev Dale,

Isn't that what Bertha was threatening?

Bill