High EGT with increased valve overlap

[Thom]

Hello,

I have a problem controlling exhaust temperature on my turbo'd engine since I have fitted a new camshaft. Engine is a 3L SOHC.

Old camshaft :
Duration : 255/257° seat to seat
LSA : 114°

New camshaft :
Duration : 272/272° seat to seat
LSA : 112°

This means I went from a valve overlap of 28° to 48°.
After advancing igntion timing by between 10° on low load to 5° on full load I was able to reduce EGT by between 50 to 100°C but I'm still now 100°C higher than with the previous camshaft, and if I really push it I am seeing an exhaust temp of ~1100°C between exhaust manifolds and the turbine inlet.

Notes :
- I am not able to cause detonation by advancing ignition timing. I notice the point beyond which advancing ignition timing causes a loss of power, but I can't hear knock through my J&S.
- I have the fuel map adjusted so that AFR at peak boost (220kPa) richens from 12.5 at 5000 rpm to 12 at 6500 rpm
- For testing I reduced cam timing by 3° and it made peak EGT even higher.
- Inlet air temp is on average about 25°C above ambiant.

I understand that the increased overlap causes unburnt fuel to slip through the exhaust valve thus increase EGT.
-> Short or installing back the standard camshaft, what options do I have with the tune to reduce EGTs?

Thanks.

Edited by Thom on Thursday 18th June 10:17

[stevieturbo]

Thom said:

Hello,

I have a problem controlling exhaust temperature on my turbo'd engine since I have fitted a new camshaft. Engine is a 3L SOHC.

Old camshaft :
Duration : 255/257° seat to seat
LSA : 114°

New camshaft :
Duration : 272/272° seat to seat
LSA : 112°

This means I went from a valve overlap of 28° to 48°.
After advancing igntion timing by between 10° on low load to 5° on full load I was able to reduce EGT by between 50 to 100°C but I'm still now 100°C higher than with the previous camshaft, and if I really push it I am seeing an exhaust temp of ~1100°C between exhaust manifolds and the turbine inlet.

Notes :
- I am not able to cause detonation by advancing ignition timing. I notice the point beyond which advancing ignition timing causes a loss of power, but I can't hear knock through my J&S.
- I have the fuel map adjusted so that AFR at peak boost (220kPa) richens from 12.5 at 5000 rpm to 12 at 6500 rpm
- I vary injection timing from 400° BTDC to 460° BTDC from 4k rpm-on.
- For testing I reduced cam timing by 3° and it made peak EGT even higher.
- Inlet air temp is on average about 25°C above ambiant.

I understand that the increased overlap causes unburnt fuel to slip through the exhaust valve thus increase EGT.
-> Short or installing back the standard camshaft, what options do I have with the tune to reduce EGTs?

Thanks.

Cam timing can effect EGT, however I'd say more overlap if anything would reduce EGT's
Unburnt fuel should help cool the exhaust valves, as would any air passing through.

Satement in bold causes concern.

Are you saying you advanced timing by 10deg, TO 5deg BTDC ? ie it was previously firing 5deg ATDC ?

Even 5deg BTDC spark timing is pretty low unless you're really pushing lots of boost.

Check your cam timing, opening the exhaust valve too early could easily cause excessive EGT's

[Thom]

No, I said I had advanced ign timing by 10° to 5° on the whole map, which lowered EGT, but it's still too high.

stevieturbo said:

Cam timing can effect EGT, however I'd say more overlap if anything would reduce EGT's
Unburnt fuel should help cool the exhaust valves, as would any air passing through.

Unburnt fuel can cool down exhaust valves, but it will still flow through the exhaust and burn in it, no?

stevieturbo said:

Check your cam timing, opening the exhaust valve too early could easily cause excessive EGT's

I did already, retarding it 3° made for a even higher EGT.

[anonymous-user]

Firstly, the temp you measure with a thermcouple in the exhaust manifold is not actually of any direct importance itself (As it is just the average temp the end of the thermocouple is heated too by the passing exhaust gas (which will always been very hot!) However, it is generally used as an indicator for the potential upheat to things that do matter, namely the exhaust line components, such as valves, turbines, wastegates, etc etc.

So, whilst 1100degC is what you would call "HOT" it is not excessive if your exhaust line can withstand the temps to which those parts are heated (which is tricky to measure, and depends upon how long the engine operates at such speeds and loads etc (due to thermal inertia) But, with what i would call "normal" components, ie not ultraexpensive competition parts, most people would be aiming for a 3mm thermocouple (in middle of entry section) TIT (Turbine Inlet Temp) of a maximum of approx 975 to 1000degC. Most turbines and exhaust components will withstand that for long periods of time.

Now, you say you are running down to around 12:1 AFR, which is not really that rich. Typical road cars will run down to around 10 or even lower in some cases at high load/rpm to limit EGT's (The extra fuel mass cools the charge due to the latent heat of evaporation, and results in a cooler exhaust gas stream as there is a larger charge mass to heat up.

Obviously, running that rich will cost a small amount of power, but generally, it's not too bad.

My suggestion, run some sweeps of EGT vs AFR at a fixed ignition angle and see how rich you'll have to go to reduce measured (stable) TIT to below 1000degC

[Thom]

Thanks for your input.

For what it's worth the K-type thermocouple I use is located about halfway between the outlet ports of the head and the turbine inlet.
The total length of piping between the head and the turbine inlet is about 1.5 meter, so if I measure 1100°C in between, is it reasonable to suppose outlet ports are seeing 1200°C and the turbine inlet 1000°C?

Exhaust tubes are made of inconel so I suppose they can withstand that much heat, but cause of concern is the longevity (or probably rather the lack of) of the turbo, and more to the point, engine internals under sustained high loads. Exhaust valves are sodium-filled but I'm not sure what it means in terms of heat transfer capacity.

[Pumaracing]

I'm not sure why you should be surprised at what you've found. You're opening the exhaust valve a lot earlier now when the cylinder pressure and gas temps are still very high. High CR normally aspirated engines extract a lot of the gas energy early in the power stroke and show a very rapid drop off in cylinder pressure and gas temp as the piston descends. Essentially this is a function of the expansion ratio of the engine which is related to the inverse of the compression ratio.

Low CR turbo engines have a lower expansion ratio, take longer in the power stroke to extract the combustion energy and the cylinder pressure and gas temp drops off more slowly.

With more duration plus a tighter LCA you've given the exhaust opening point a double whammy. You can now see why increasing ignition advance and starting the burn earlier has dropped the EGT. Retarding the cam timing would do the same.

[Thom]

As odd as it may sound and as I mentioned retarding the cam timing by 3° made it worse.
However I'm beginning to feel that is partly because my turbine is slightly restrictive and TIP may get higher while trying to shift peak torque at higher rpm by retarding the camshaft...?

Anyway I added a gear/EGT-based fuel trim table and was able to hold temp below the 1000°C mark. May not be the cleanest way to proceed but that will do until I fit a larger turbine housing which is anyway in order. Didn't ever expect I'd be pleased to reduce her TITs...!

Edited by Thom on Thursday 8th May 20:27

[Mojocvh]

That was very interesting, thanks.

Mo