Is knocking/detonation possible in gaseous fuel fired Otto engine? 1

[ebarba]

Hi all!

We're running an 8.0L turbocharged engine on wood gas to drive a stationary electric generator, rated 100 kW.

Wood gas composition is essentially:
20% H2
20% CO
5% CH4
55% N2

We are experiencing loud "clanking" noises coming from the engine when we approach rated power, the conditions of operation are as follow:

Engine compression ratio: 9.5:1
MAP: 165 kPa (24 psi a)
Mixture type: Very lean (sorry, no exact value as we're using a narrowband O2 sensor)
Coolant temp: 90ºC (194ºF)
Engine speed: 1500 rpm
Power at the engine shaft: 93 kW
Spark advance: 12º BTC

The knocking (or supposed so) develops when we try to increase the AFR to push further the engine towards the expected power target.

I thought it was "impossible" to experience knocking with gaseous fuels, especially with Hydrogen in the mixture and so small an advance. On the other hand, the overall compression ratio (considering boost) is quite high and probably the engine is running a bit too hot.

Spark plugs do not show evidence of knocking or detonation.

Are this noises really knocking?
We will try again with lower advances (down to 5º), is this a good idea?
We will also try with richer mixture from the start (near stoich) next time, keeping it that way while increasing the power. We hope we can reach the target power in this way with a lower boost from the turbo, hence lowering the compression ratio.

Eagerly awaiting for your feedback!

Thanks and regards!
EB

 

[ebarba]

This is what we learned today:

We can have 50kW (50% power), with a very smooth running engine and a (very-difficult-to-hear) rumble at the following conditions:

1500 rpm
125-130 kPa
Slightly rich mixture
99ºC inlet temperature (very scary)
20º advance

If we decrease advance to 7º, the engine will loose power delivering 25kW and become very rough, the rumble becomes more noticeable and the engine starts jerking.

Returning the timing to 20º advance, engine goes back to the initial conditions on this post, running smooth.

Is this pre ignition?
- when we advance we allow the spark plug's combustion front to reach all the combustion chamber before preignition starts (at the cost of loosing power)
- When we retard the preignition creates the second front and pressures skyrocket?

Is this possible?

 

[Lou Scannon]

Combustion Analysis

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Why the increase in charge temperature? On 17th Aug you said the charge temp was 80*C at 165 kPa MAP. The higher MAP should produce a higher CAT.

Is your fuel gas hot?

If this turns out to be an octane rating issue, you may need to consider one of the following:
- Decompress the engine.
- Intercooling.
- Water injection.

je suis charlie

 

[tbuelna]

One thing that might be complicating things is adding fuel ahead of the turbo compressor. This will affect its performance.

Based on your posts stating that this 8.0L engine operating at 1500rpm on around 9psi boost and experiencing detonation while producing only 25kW, seems like it is probably running extremely lean.

 

[gruntguru]

Yes, if the 9 psi is at the intake ports (not prior to a throttle plate) it would be very lean indeed. Flame speed would be so low, it would need a bunch of ignition advance. Sounds counterintuitive but with the optimum advance you would have to throttle it back a lot and end up with MAP below atmospheric to make the 25 kW (and no knock).

If I was tuning this thing I would want to know the AFR. Buy or borrow a wide band lambda system. Start at small throttle (say 60 kPa MAP) and adjust the lambda to 1.2. Vary the timing for best output with no knock. If best output occurs over a range of ignition advance err on the low advance side. You can try going richer or leaner from here to see where it is happiest.

Repeat at 70 kPa MAP etc.

je suis charlie

 

[tbuelna]

The situation described in the most recent post does not make sense. I don't see how retarding the ignition timing from 20deg to 7deg at part load and 1500 rpm, while changing nothing else, would result in a 50% drop in BMEP and rough running. You would likely see a drop in BSFC, but as long as the A/F ratio remained "slightly rich" and the inlet air stayed around 99degC, I don't think you would experience detonation that did not occur at the 20deg ignition timing point.

 

[gruntguru]

tbuelna. Perhaps there is something else changing in response to the drop in power - a governor or fuel control.

je suis charlie

 

[ebarba]

I think we finally found exactly what was happening, during some tests over the weekend. As tbuelna points out, it didn't make sense at all, so we re-checked everything from ground up. We found a blockage in the gas producer, which we think explains everything:

At 165 kPa and lean mixture the engine WAS knocking: the pressure and temperature in the cylinders where so high that a small increment in the fuel quantity led to detonation. Ideal gas, polytropic conditions calculation throw 750ºC and 44 bar at the end of the compression stroke, which is more than enough to auto-ignite hydrogen.

The above is confirmed by a normal knock behavior: when it was knocking and we retarded, the power increased and the noise went away. Also, working at such a high pressure improved the cycle efficiency, so less gas than usual was sufficient to reach the same power, as compared to stoich condition. As per logic, it was however very little power (30% of rated output).


At 125 kPa and stoich mixture, what was happening is that we had less fuel, less air, less efficiency... We wrongly assumed that the jerking, rumbling and decrease of power was knocking, when it was just plain and simple MISFIRE, which got worse by retarding, obviously.

What got us completely out of way was that the butterfly was wide open and we had stoich mixture, so we assumed full power could be achieved and that we were missing something in the tune-up. We were getting about 60% fuel, so to compensate for that, the mixture control decreased the airflow to meet our "stoich" request, even though we were indeed at WOT.

Something worth noting is that air control for mixture adjust is upstream of the throttle body and can virtually close completely the airflow, this is more or less common in wood gas systems. We never suspected that the air control was making such a large decrease in air flow, not knowing of the restriction in the gas producer.

We are now fixing the problem in the gasifier and I expect all will be fine afterwards. In any case, comments and challenges to the above reasoning are always welcome!

Thank all of you guys for helping!!!

 

[gruntguru]

Just goes to show, it is hard to give advice without knowing the layout of mixture/fuel/air controls and where the pressures stated are being measured.

je suis charlie

 

[ebarba]

Gruntguru, about the boost pressure, it's being measured downstream of the throttle body, at manifold's inlet.