Tune your LPG system to avoid detonation

Untuned LPG Systems can cause detonation and damage to a head

It has been seen, in this example in a Nissan Patrol TB42 cylinder head, that after fitting a replacement head oil has been found in the cooling system. Even after only being on the vehicle for a short period of time (2,000km's), there was considerable oil in the coolant. In this particular example, the vehicle was dual fuel, both Petrol and LPG. 

The cylinder head and gasket were removed for inspection.

When inspect it was found that the head was bent around 0.010". There was also significant evidence of severe detonation (uncontrolled combustion). Detonation in this case had caused the cylinder head to melt around the combustion chamber fire ring area which then lead to the head gasket failing. That's where the oil got into the coolant. 

After digging further, it became apparent that the LPG system on the vehicle was not in serviceable condition. The owner had been instructed not to use the LPG system but this had been ignored. 

Irrespective of whether the system is tuned properly or not, LPG creates a much hotter combustion environment that in turn produces higher cylinder head temperatures. If the system is not tuned properly and is detonating, it is likely the head will melt. This is exactly what happened in this case. 

It was found that the LPG system was the cause of the detonation due to being out of tune. 

Of course it's vital that any fuel system remains properly tuned but this is especially important with an LPG engine. Detonation is caused by a variety of things in an engine but it's more commonly caused by incorrect ignition timing, too much ignition advance or the air/fuel ratio being incorrect. The combination of these things together can cause severe detonation. 

When you're trying to tune a car for Dual fuel it gets even harder because you have to handle both petrol and LPG characteristics. 

When it comes to LPG ignition advance tuning, it's different to petrol due to burn rate. At lower RPM, the burn rate of LPG is slower which means you need more ignition advance. At high RPM the LPG burn rate is much faster, so less advance is needed. If you apply a fixed advance rate, or the same advance as petrol, you'll have problems. 

When it comes to petrol it's actually the reverse being true. Less advance is required at low RPM and more at higher. Therefore an engine running dual fuel needs two ignition maps. Apply one to the other and you'll have detonation. 

To add to this, if your air/fuel ratio is too lean, you'll increase the speed to combustion which also impacts the advance required. 

In this particular example there was no reported damage to the bottom end of the engine. This is very lucky as normally there is substantial damage to rings and pistons when detonation occurs.