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XPAG Engine Temperatures and Cooling

This article can also be read in conjunction with :

XPAG Water Flow–Cooling.
Overheating or Vaporisation.
Warming Up times.


XPAG Engine temperatures and cooling


Alas, only about 23% of the petrol your engine burns actually gets to power your car. That is only a quarter of each gallon does any work. In fact by the time that the mechanical losses are taken into account, less than 15% of it gets to the rear wheels and shoves the car along. Do not fret as the steam engine as used in locomotives on railway lines can get an efficiency as low as 5%, so out of every 100 tons of coal just five tons does anything useful! Do not let anyone fool you that electric motors any any more efficient as the electricity they use comes from fossil fueled power stations. Yes, they may get 95% of the electricity to do useful work, but that power station burns 100 tons of coal to give you just 10% efficiency overall.



Back to your little car’s engine. Here we are dealing with technology of the 1930s in your Y Series, so today’s car engines might just be a few percent more efficient but they each still produce about 3 tonnes of CO2 a year. The diagram is actually of a 1140cc Morris Ten Series ‘M’ engine. Like the MGs ‘XPAG’ engine, this is from the same stable and is also a ‘X’ series engine. That in the MG is just a bigger, stronger version. By far the hottest part of your engine is the exhaust valve’s head at 760 degrees C. This gets rid of its heat by soaking it into the valve seat which is still pretty hot at 350 degrees C and via its valve guide. If it did not it would soon glow a cherry red colour and explode the fuel too early. The burning air and fuel inside the cylinder reaches 2,500 degrees C and the piston has to be cooled by the engine’s oil as well as soaking heat into the cylinder walls. It still runs at 300 degrees C though. The cooling water flowing through the cylinder head leaves at around 80 degrees, though around that exhaust valve seat it must be almost boiling. The water is shifted at quite a speed so as to not stagnate and boil. (All these temperatures are in degrees Centigrade, to put them in Fahrenheit would give you a heart attack.)


The engine gets rid of the excess heat in four ways. Most of it goes out of the exhaust and some is lost from the engines outer surfaces. Of the rest the water and oil deal with about 50% each. Yes, your oil does a lot of the cooling as well, just look how hot the big-end and main bearings run. The oil is designed to run hot. The oil in the sump loses its heat via the sump casting and returns to the oilways at about 85 degrees; the water is passed through the radiator and returns at about 50 degrees. Some heat is returned to the engine by the inlet mixture, heat is removed from the ‘hot-spot’ in the inlet manifold, the ports in the head and from the inlet valve. This is a bit of a compromise as too warm a mixture is less dense than a cold one, but fuel evaporates better in warm air. Cool, dense mixtures give more power.




The cooling water enters the head from the back on the XPAG as that is the ‘hot end’ of the engine. The front end gets cooled by the incoming air so it is logical to cool from the back first. To keep expansion and temperature differentials that cause cylinders to warp about equal, the block itself has almost no water flow other than the action of thermosyphon, ie hot water rises. The water pump assists the water around the system but is at the outlet side of the radiator, that is it pumps the cooled water into the engine, not the hot water from it; better for a longer pump life. Now, if grey-cast iron has a melting temperature of about 1500 degrees C, just how does the exhaust valve seat in the head survive? Now you are worried. It is very important to remember that the exploding fuel/air mixture burning at 2500 degrees lasts a split second. As the expanding volume pushes the piston down, so the temperature drops as does the pressure (these two are interlinked by laws of physics) and by the time the exhaust valve opens the temperature is much lower at around 1000 degrees. This is still hot enough though for the cast iron to start to burn away, and that was one of the things that Tetra-Ethyl-Lead helped to stop by coating the valve seats with a thin layer of lead, a by-product of its improving the octane rating of the fuel. That is why once that lead ‘gasket’ on the valve seats has gone, you will soon require hardened steel valve seat inserts. Before TET was used manufacturers automatically fitted hardened seats, but TET meant they could save money and just leave the cast iron to cope. It was the long fast runs on motorways with an engine never designed for such use that soon showed up the vulnerability of those exhaust valve seats. In all probability if you keep to ‘B’ road speeds and off motorways, your valve seats may last for years before needing attention. Fast running for long periods raises engine internal temperatures to very high levels, especially if your radiator is past its best. The Y has a radiator with brass header and bottom tanks, copper water down tubes, but mild steel fins. These fins eventually corrode and expand, and block up the air passages, the result is a boiling engine. If you live in a hard water area then lime scale will slowly block up those copper water tubes as well and this is not easy to see.


So, whilst your engine may have many split-seconds of 2500 degrees C, the incoming charge, the oil and water cooling, the losses out of the exhaust and the heat radiated from the castings keep the average temperature low. But even then that little diagram still makes you think, does it not?
























Article supplied by Neil Cairns