Steve Wood wrote: > I have a custom-made heat exchanger fitted onto the exhaust > of the air-cooled Lister HR2 on Bream, used to produce > domestic hot water. The heat exchanger has 11 pipes of 1/2" > or 1" diameter and the rest of the exhaust is the standard 2" > diameter so the total cross-section area is actually greater > in the heat exchanger but clearly there is resistance caused > by the baffle where the pipes start (if this description > isn't clear think of a steam train, it's quite a similar concept.) > > It has been suggested that problems we are seeing with the > engine are caused by back pressure due to reduced effective > diameter.
Is there a reasonable sized chamber at each end of the heat exchanger to allow the exhaust to get from the 2" pipe to the 'fire tubes' and then back into the 2" pipe at the other end? If so, I can't see this being the cause of the problem. You could confirm this by temporarily replacing the heat exchanger with a length of plain 2" pipe and seeing if that solves the problem. Otherwise, are there any tight bends in the exhaust system which might be causing back pressure, or is the exhaust sooted up? > There is loss of oil from pretty much every orifice > of the engine, typically a pint a day. Its not labouring or > losing power, though the engine room is very hot as the > cooling air is not ducted out. Oil pressure is steady at > 20psi, which is normal. When Fulbourne had an SR3 the cooling air was ducted out through the pigeon box (which had to have the flaps open when the engine was running). It used to put out a lot of hot air - you could dry wet clothing pretty quickly by draping it over the top. Sounds like your setup is recycling warm air, and that could be the cause of your problems. Is your engine running hot? Do you have a temperature gauge? Does the engine run notably cooler if the engine room doors are open and there is a reasonable crosswind? > The question is, is it the heat exchanger causing the > problem, or the lack of ducting, or is more likely to be > something else altogether? My inclination is to go for the ducting. Can you rig up something temporary to see if it helps? One other possibility is that the crankcase is being pressurised, either because the breather pipe is blocked, or because you are getting leakage past the piston rings. This could cause oil to be pushed out of all sorts of places it shouldn't. I had this problem on a Petter PH1 on a dumper truck. The oil was horribly black, and didn't look to have been changed in years. The breather pipe from the crankcase to the rocker box was completely full of a thick black oily gunge and crankcase oil was being blown out of the oil filler and also into the clutch bellhousing. A good cleanout of the breather pipe with a bit of cloth on a length of wire together with the addition of an oil additive that is supposed to dissolve gunge in the engine sorted out the immediate problem, followed shortly afterwards by an oil change. David Mack
