What is the reason why a single-cylinder diesel engine can be cranked without decompression?

[TaliaRose]

Pressurization valve may be malfunctioning. Here is an introduction to the pressurization valve: 1. The pressure reducing valve regulates: reducing the inlet pressure to a required outlet pressure, and relying on the energy of the medium itself to automatically maintain a stable outlet pressure. 2. From a fluid dynamics perspective: the pressure reducing valve is a throttling element with variable local resistance, meaning it changes the throttling area to alter flow velocity and fluid kinetic energy, resulting in different pressure losses to achieve pressure reduction. 3. System regulation: balances the fluctuation of downstream pressure with spring force, maintaining downstream pressure within a certain error range.

[LeAlaina]

I remember when hand-cranking an old single-cylinder diesel engine, if it feels unusually light without using the decompression lever, there's an 80% chance it's an air leak somewhere. Just like we often encounter when repairing tractors – poorly sealed valves are the most likely culprit, whether it's due to a burnt section or carbon buildup jamming them. If white smoke is puffing out from the crankcase, it usually means the piston rings are worn thin or the cylinder liner is scratched, allowing compression gases to leak straight into the oil pan. Once when I disassembled an engine, I found the cylinder head gasket blown through by high-pressure gas, with coolant mixing into the engine oil, making the hand-cranking feel extremely light. Also, aging gaskets around the fuel injector mounting holes can cause air leaks – you can spot bubbles by smearing soapy water on them with your finger. These issues must be fixed promptly; insufficient compression makes the engine weak and fuel-thirsty. If you notice these symptoms, check the sealing system immediately.

[McBeckett]

From the perspective of thermal efficiency, poor compression in single-cylinder engines: the core issue is combustion chamber sealing failure. Valve components have the highest failure rate—bent valve stems cause delayed closing, and loose valve seats result in continuous air leakage, leading to cylinder pressure measurements below 18 kg/cm². When piston ring side clearance exceeds 0.2mm, high-temperature gases can escape into the crankcase through the ring grooves, accompanied by a sharp increase in oil consumption. Some overlook the sealing of the fuel injector installation hole, but in reality, high-temperature deformation of copper gaskets can also cause pressure loss. Cylinder head gasket erosion is the easiest to identify, often causing bubbles in the radiator. Such faults cumulatively lead to cold start difficulties, and it is recommended to use a cylinder pressure gauge for segmented testing.

[AnnieFitz]

Back when I was reclaiming wasteland, my 195 diesel engine had the same issue - the crank handle spun like crazy without needing to depress the decompression lever. Most likely it was valve leakage; upon disassembly I found black carbon deposits stuck on the exhaust valve. Could also be piston ring end gaps misaligned - last year when replacing rings I didn't pay attention to gap positioning, resulting in constant blue smoke during operation. If the radiator keeps bubbling, it's almost certainly a blown cylinder head gasket. Older engines also tend to leak air from injector seat gaskets - you can hear hissing when touching it. All these problems make the engine hard to start and perform weakly during work.

[LaDaisy]

Piston assembly failure is the main cause: When the elasticity of piston rings deteriorates, an end gap exceeding 0.8mm can lead to blow-by rates surpassing 30%. Particularly after wear on the top compression ring, compression pressure leaks through the ring land into the crankcase, resulting in noticeably weaker pulsation during hand cranking. When oil ring return holes become clogged, oil scraping efficiency decreases, causing engine oil to participate in combustion and generate carbon deposits, further compromising cylinder sealing. A case was encountered where cylinder liner out-of-roundness exceeded 0.05mm, preventing piston rings from properly contacting cylinder walls during cold starts. It is recommended to inspect ring end gaps every 500 operating hours.