A generator set that will not reach its rated load, runs hot and smokes under load is often blamed on the engine. Quite often the cause is behind it: an exhaust system that resists the gas more than the engine was designed for. Back pressure is the quietest way to lose output.
What the limit means
An engine pushes its exhaust out against whatever stands in the pipe. The engine maker states the maximum pressure the engine may see at its exhaust outlet at rated load. Below that figure the engine performs as rated; above it the cylinders do not clear completely, the charge air suffers, the exhaust temperature rises and the output falls. Turbocharged engines are the most sensitive, because the turbine sits in that gas stream.
Everything counts
The back pressure at the engine is the sum of the whole route: the silencer, the straight pipe, every bend, the expansion joints, the flexible connector at the engine, the rain cap or the stack outlet. Each adds its share, and the share grows with the square of the gas velocity - a pipe one size too small costs far more than its size suggests. A silencer of a higher grade adds more; a longer route adds more; a tighter bend adds more than a sweeping one.
Where it goes wrong
Most exhaust systems are calculated correctly on paper. They go wrong on site: the stack is moved to clear a window and gains two bends, the pipe is run around a beam, a second rain cap is fitted, the silencer is swapped for a quieter one after a complaint. Each change is small; together they can take the route over the limit. The second common cause is time: soot and scale narrow the pipe and load the silencer over the years, so a system designed exactly at the limit drifts above it.
Designing with margin
A sound exhaust design keeps the total well below the engine limit, so that fouling and small site changes stay inside the margin. Where the route is long, the pipe diameter goes up rather than the attenuation down; where a quiet silencer is needed, the route is kept short and straight. The calculation is repeated whenever the route changes - a question the installer should ask before moving the stack, not after.
Checking an existing installation
An exhaust system can be checked: a pressure measurement at the engine outlet at full load, compared with the maker's limit, tells whether the set is losing output to its own exhaust. If it is, the remedy is usually a larger pipe, fewer bends or a silencer with lower pressure drop at the same attenuation - and sometimes just the cleaning that has been postponed for years.
AXON Technic designs complete exhaust systems for generator sets - silencers, piping, expansion joints, insulation and stacks - and checks the back pressure of the whole route with every selection. Send the engine data sheet and the planned route through the contact page.