Engine Failure

The engine temperature is slightly high, and the oil pressure is also high. Normally a warm engine would thin the oil and drop the pressure, and I’m low on power. With hindsight its obvious what’s happening. The oil has thickened to the point where it’s resisting the engine from working.

But at the time I don’t know so it could be anything. I stop and change the radiator coolant. No effect. I stop and remove the exhaust filter which might be blocked. No effect.

The temperature is rising and settles just under 100C. I remove the thermostat and clean the outside of the radiator. No effect. I check the connections to the gauges. No effect.

We stop for dinner, and when I drive off again, the oil pressure drops to zero. That’s bad, really bad.

I check the dip stick to see if there is any oil left. It’s there, but it’s turned to a rubbery tar, and is stuck to the dipstick like silicon or chewing gum. I drive on to the next filling station which thankfully sells oil, open the sump and nothing comes out. It’s all gone solid.

Finally it all makes sense. Engine oil reacts with vegetable oil, and somewhere in my engine there is a leak. It’s one of the reasons I check the oil every morning for level and thickness. I’d been worried about it thinning as it mixed in with fuel, but over the last few days I’d been reassured that it looked thicker, and put the thinness down to the variability in morning temperature.

I’ve been changing the oil every 6000km but this time decided to leave it til 10000km as I’m using an engine oil which is supposed to avoid this problem. After 8000km it polymerised. It’s hard to imagine what a mess polymerised engine oil is. It’s heat resistant and rubbery, and it sticks to metal like silicone sealant.

That night I managed to flush enough of it out with diesel. It will dissolve a bit in diesel. I then ran the engine with a mix of new oil and diesel to try to dissolve the rest, but after 500km arriving in Delhi the engine was still full of it, and, at some point it had already caused one of the pistons to cease.

So here in Delhi I’ve had no choice but to do a complete engine rebuild, with new pistons and cylinders. It’s major surgery. You can’t strip the engine back any further than this.

Thankfully it turns out that this engine was copied by Tata and is very popular here, but they copied it with a few modifications so not all the parts are the same. The big problem has been the pistons. The Tata pistons are a little taller than the Mercedes ones.

Pistons aren’t cylindrical. They are very slightly oval. They are cut on an OCUMA (Ovality Cutting Machine). It’s like a lathe, but as they spin the cutting tool vibrates slightly in and out creating an ovality which when in the engine means that that the piston deforms into a cylinder when forces of the exploding fuel act on it.

So unlike most things in India, you can’t get a backstreet mechanic to make a new one. But I can get the machine shop to modify one that is almost the right size.

Shaving 2mm off the top of the Tata piston will create a piston that will fit, but the risk combustion bowl in the top of the piston. When the piston is at the top of it’s cycle all the air is squeezed into the small bowl cut into the top of the piston. The volume of the bowl is critical because it dictates the compression ratio. The cylinder is one litre, and the bowl is about the size of a shot glass, so all that air is compressed down into that space before the fuel is added and for the engine to work properly the compressed air has to be the right pressure.

It’s hard to measure these things accurately but the bowls are almost the same size, however worryingly the new bowl, in the Tata piston, when cut down will be about 5ml smaller, which means the pressure increase will be a bit higher. This might affect the engine in a number of ways. Firstly the Injector pump might not have enough force to overcome the increase in pressure, so it won’t push any fuel into the engine. Secondly compressing air like that creates heat so the extra compression might mean the engine runs hotter than usual. Finally it might be too much pressure for the bearings or the crankshaft to take, and either might crack under the strain.

On my phone's calculator I estimate that with the original pistons the air is compressed about 24 times, so the pressure is 24 times atmospheric pressure. With the cut down Tata pistons it will be about 27 times. I’d really like a syringe to measure the volume properly.

To my mind this difference from 24 to 27 is nothing, and if anything will make the engine run a bit more efficiently, but the mechanics are petrified that it won’t work, and it can’t be tested other than by reassembling the engine which is a day’s work.

The guy in the parts shop looks at me horrified when I tell him what I plan to do. “Modifying pistons is not recommended” he tells me. No shit, Sherlock? Is that right? At first I’m stuck for words. Then I start to think of all the things I’m doing which aren’t recommended. Crawling around under my bus on some dirty street isn’t recommended, riding across Delhi in the back of a scooter without a helmet isn’t recommended, not taking anything for the cough I’ve caught off my mechanic isn’t recommended, and that's just this morning. Driving to India in a 21 year old shitty bus, dragged out of a scrap yard, and fuelled by all sorts of crap I’ve scavenged along the way is not recommended. I think increasing the working pressure of the engine from 24 bar to 27 is probably the least of it.

I think it will be fine, but I’ve got enough people filling me with fear it’s really put the doubt in my mind. So now I want to do it just to see if it will work.

The engine will either not run, not run for long, or I’ll spend the next 6 months being highly suspicious of every characteristic of the way it runs, then forget about it, then in 2 years time I’ll remember the pistons and realise it was all Ok.