So I have a single cylinder diesel engine. We do not use it to produce power (it is operated by an electric motor) but to measure the pressure at low rpm (100-200). The volumetric compression ratio is around 21.5 and the dynamic CR should be very near to that since the valves open and close near the TDC or BDC. I noticed that the pressure at the end of the compression stroke is much lower than expected, despite accounting for the fact that some air will always leak resulting in lower pressure and found the cause to be bleed/leak from the exhaust valve.

During compression, a significant amount of air is pushed out of the exhaust through the valve and during expansion (power) the piston draws in air from the exhaust valve while the valve is still closed. I need to achieve as much pressure as possible inside the cylinder at the end of compression stroke. So, I was thinking if there is some way to control this air leakage from the exhaust valve. Right now, I am thinking of (1) applying paint or silocone primer to the valve and seat, or (2) applying gasket maker sealants (RTV??), or (3) attaching a thin rubber gasket with a strong adhesive.

I know these are not ideal options, but I would like to generate as much pressure as possible, even if it means some damage to the engine. Any suggestions would be greatly appreciated. Thanks.

PS - Using maths, the max pressure should be around 70 bars (theoretical, lower in reality) but I am getting around 20 bars (EDITED, previously 11 by mistake) at 100 rpm. The pressure increases to 35 bars at around 1200 rpm and does not go any higher.

Edit - I tried lapping the valves and when that did not work, I ordered new cylinder head and new valves. I got slight improvements (11 bars from 7 bars) but this is no way near to what I want. So bottom line, what I need is high pressure at low rpm. Difficult, but let's hope.

Edit - When changing the cylinder head and valves, I changed the head gasket too, and I know the exhaust valve is the main culprit now because you can hear and feel the air being pushed out and sucked through the valve when the valve is closed.

  • So this is an cylinder that does not contain explosions? I imagine there is heat from compressed air, so what temperatures are we talking about on the valves and seats?
    – JPhi1618
    Commented Dec 1, 2017 at 16:31
  • Yes, there are no explosions in the cylinder. We don't have a temperature sensor, only a pressure sensor. However, if the pressure were higher, I would expect the temperature to be around 650 - 700°C, but right now it should be much lower at around 100°C. Commented Dec 1, 2017 at 16:40
  • Well, then that's the point I was getting to. If you expect a perfect seal to give you temps that high, you need a material that can handle the heat. Paint or RTV isn't going to cut it. Bad compression is a real problem on engines so why not fix this the same way rather than some sealant?
    – JPhi1618
    Commented Dec 1, 2017 at 16:45
  • There are some paints (or silicone primers) that can withstand up to 650°C, and also some rubbers. The thing I am worried about is the gasket (or paint) breaking up and falling into the cylinder as the valve opens and closes. So, which way of fixing the bad CR are you referring to? I forgot to mention that I have already done lapping on the valves, did not work. Then I ordered new cylinder head and valves, a slight improvement but not even close. I will add these details to my question. I know low rpm could be the main problem, but that is exactly what we need, high pressure at low RPM. Commented Dec 1, 2017 at 16:51
  • Well, the types of fixes I was thinking about seem to be what you have done already. There may be more, but its beyond my expertise. Just as an added detail for others, how durable does this need to be? Does it need to last for an hour to perform a quick experiment, or is this something that will be run for days at a time at constant duty?
    – JPhi1618
    Commented Dec 1, 2017 at 16:55

3 Answers 3


if your exhaust valve has the proper gap and is perpendicular to the valve guide without excessive play in the guide with correct seat pressure for the valve ..have you tried squirting a little oil into the chamber to see if it raises the compression..ie ring check..


I would not use any kind of "sealer", it is a temporary aid and once you increase the pressure then they will probably fail.

Sounds to me like you need to do the following:

  • Increase the pressure used to close the valves such as valve springs with higher seat pressure.

  • Increase the amount of time the intake is open to more fill the cylinder.

  • Ok, I will explore the option of using a spring with higher spring constant, but I am not sure it will take me to where I want to get. And the intake valve timing is, I would say, optimum for this engine operating at low rpm. Thanks for the answer. Commented Dec 1, 2017 at 21:23

If you want this thing to function as air compressor cam timing will need to be changed, i.e. you have little intake inertia effect that exists at higher RPM. No valve timing overlap at tdc will make a big favorable difference in your application.

  • Thanks for the answer. There is virtually no valve timing overlap. The clearance volume is cut out in the piston itself, so the gap between TDC and the cylinder head is really really low. For that reason, the valve timing is optimum for the engine. Also, what I am aiming for is high temperature which is brought by high pressure. Commented Dec 1, 2017 at 21:20

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