Ignition timing cause & effect on EGT/OIL temp

I am trying to wrap my head around the relationship of ignition timing and its effect on EGT/Oil temp, this is on a IO390 engine with Electronic ignition advancing the timing and at around 4-5k AGL, so not high up.
Some tests has shown little effect of advancing beyond 23-24 BTDC at these level altitude. My test shows when I change my advance from a max of 24 BTDC to 30, I see a slightly higher oil temp but lower EGT (not touching the Manifold or mixture), I also pickup one to two knots, more often two.

Just though if some smarter people than me can break down what is happening here, cause and effect

Cause and effect - always a good thing to examine.

Here you can look to the effect on EGT when doing a mag check - turn off one mag (or EI) and all EGT will rise. This reflects that the effective timing of the engine is retarded when running on one mag or EI. If you did this long enough you would also see a drop in CHTs.

Similarly, advancing timing (within limits) should lower EGTs but raise CHTs. You are getting more heat work out of the fuel you are using. So for the same fuel flow you are getting more power - as reflected by your measured increase in speed. Higher CHTs should also be reflected with higher oil temp.

Again - within limits. I have my ignitions set for a max of 34 degrees advance, and this advance only happens at lower manifold pressure (as in high altitude flying). This is for a 180HP parallel valve IO-360. For your engine I suspect a top advance of 29 degrees is more standard.

Note - when LOP the engine transitions from excess fuel (ROP) to excess air as the cylinder head cooling agent (other than engine cooling air). For me LOP yields lower CHTs - reflecting the reduction of actual produced power. Power per unit of fuel burned however goes up.

Carl

Carl,
I am with you on all of this, both based on experience and logic, as I often pay attention to my EGT during mag check. What puzzles me, the higher advance (30 compared to 24) brings the EGT down and raises the oil temp.

Mehrdad, give me a call.

The angle valve engines cool really well, so having more advance than recommended doesn't always show up as high CHT's.

With your IO-390, your timing should be 20* BTC.

If you move from 20* to say 25*, your CHT's will go up and your EGT's down.

If you move from 20* to 18*, then your CHT's will go down and your EGT's will go up.

That's it, in a nutshell.

This is a p-V (pressure -volume) diagram for a research engine at 1200 RPM, borrowed from Taylor's Internal Combustion... The X-axis is cylinder volume, with TDC on the left. The Y-axis is cylinder pressure. I've added some notes; the only variable is ignition timing.





Note that when ignition is delayed, peak pressure is reduced. However, pressure later in the stroke, around exhaust valve opening, is higher. Cylinder volume is the same at exhaust valve opening for any ignition timing, so higher pressure simply means higher gas temperature.

Conversely, advance the ignition, and peak pressure goes way up, so gas temperature is up, and CHT is high. Pressure at exhaust valve opening is lower, so EGT is lower.

Pressure and temperature are proportional when temperature is expressed in degrees Kelvin. Play with the numbers. Assume an EGT shift of 100F, from 1300F to 1400F, or from 978 to 1033K. The required pressure increase at valve opening is only about 5%.

It's often heard that "advancing the ignition gets more heat energy out of the fuel", but complete combustion, a chemical function, nets all the heat regardless of timing. This a recip engine, so it is crank angle vs pressure which has an effect on torque. Timing mostly adjusts the point of peak pressure to an overall more advantageous crank angle.

Try this thought experiment...adjust timing too far advanced. Power goes down. Is advanced timing somehow releasing less energy, or merely doing a poor job of converting it to rotary motion?

Increasing the timing will decrease EGT and increase CHT. advancing the timing at least up to the optimum level, causes the combustion to do more actual work, due to optimizing, or increasing, the peak cyl pressure from that combustion. The earlier timing also leaves the hot combustion gas in the cylinder longer (in addition to providing more piston down force and therefore increased heat due to a higher peak cyl pressure) and therefore more of it's heat transfers to the piston and the head (raising their temps) and the gas is therefore at a lower temp when it leaves the cylinder and passes by the EGT probe.

I would expect at least a couple knots from 25->30* if at peak or LOP. Its not surprising that oil temp goes up, as both the pistons and the cyl walls get hotter with advanced timing, as well as the head. The piston is cooled by the oil and the walls partially cooled by oil. Meaning heat is transferred to the oil. The greater the combustion temps, the more heat is transferred. Piston squirter engines will likely see a greater spike, as the oil is doing more of the cooling on those engines.

Larry

The biggest effect of timing on EGT is due to the finite burn duration. If timing is retarded, the end of the burn cycle gets close exhaust valve opening, meaning hot gas expelled. The more advanced the timing, the more adiabatic cooling there is before valve opening.

Everyone is talking CHT's and EGT's in the above, as did I.

You also asked about oil temperatures.

If your CHT's are running hot, then there is a good chance so is your oil temperature. The reason is that oil is used to lubricate and cool the valves and on some engines, they have squirters that spray oil on the bottom of the pistons to cool them. The oil absorbs heat that should be released in your oil cooler.

Fun fact, Lycoming had ignition set to 25 BTDC on 200HP engines. Later on the same engine they certified them (timing stamped in the data plate) 20 BTDC. Same engine.

So A&P's who have customers complain of high CHT/EGT on certified planes can't arbitrarily change timing on the 25 BTDC engines back to 20 BTDC. They have to follow the specification. However the A&P might use "parallax" and other errors when setting timing to reduce advance with in tolerances. However we have freedom to set 20 BTDC.

Electronic Ignition (experimental) allows advanced timing way past 25 BTDC. This is of course to be done only at lower power settings (way under 75% power) where CHT, EGT and OT should be lower. Detonation margins at say 55% power are such you can't detonate regardless of timing (or at least detonate or pre-ignite where it damages engine).

With all these hot rod experimental engines with higher than stock compression you have to accept the fact you are going to make way more heat and have less CHT and detonation margins.

What happens in the combustion chamber . . . .

Advancing the timing, think about what that does. First, the beginning of spark begins earlier with respect to TDC. This means the burning charge will be in the cylinder longer before TDC, causing a longer exposure to higher temperatures for the piston, cylinder walls, and head. So, more heat rejection and hotter parts. BUT, since the peak pressure is higher, and peaks right at TDC, (earlier) then the expansion ratio (at exhaust valve opening) is higher. The higher expansion combined with the higher heat rejection yields lower EGT. A better expansion ratio improves conversion to work (power) and higher speed. But obviously at the cost of higher internal temperatures, less tolerance to detonation, and higher physical loads on the piston, pin, bearings, cylinder walls, heat etc.

All that heat goes somewhere, and the longer exposure to higher temps in the combustion chamber (piston included) increases the oil temp via the oil cooling jets under the piston crown.

The engineering version is the Pv diagram shown in Dan's post.