09/29/2021
If you’ve ever been in plane, train, or automobile, chances are it had at least three of these moving up and down inside its engine (and probably many more). The piston, being driven by the crankshaft via a connecting rod, is used to compress the fuel-air mixture. The fuel-air is ignited, forcing the piston back down, this is referred to as the power stroke.
During the power stroke, the combustion pressure causes stress on the piston. However, at high RPM’s the inertial loads exerted on the piston cause even greater stresses.
At a 1000 RPM idle, a piston will cycle up and down 16.7 times every second. Assuming a 2” stroke, the piston will experience inertial loads of 43 g’s (see equ. for acceleration and plot of accel vs. RPM). While 43 g’s sounds high, this only increases the compressive stress by 1 MPa. At 5,000 PRM the inertial loads increase to over 1,000 g’s, significantly increasing the compressive stress to 117 MPa. High performance automotive engines can reach 10,000 RPM. At this point the piston is achieving a max speed of nearly 90 mph and will experience inertial loads of 4,260 g’s. The stress on the piston has now more than doubled with no increase in combustion pressure.
This piston, constructed from a low carbon steel, is now reaching its stress limit. This design would most likely experience a fatigue failure around 12,000 RPM. Pushing the RPM to 15,000 (a level only seen in F1 engines from the normally aspirated V8 era) the material yield strength has been exceeded, and failure would be expected after a low number of cycles.
This is part 1 of a 2-part series. Part 2 will examine fatigue failure criteria and the cyclic loading caused by the pistons reciprocating motion. To discuss this post, leave questions in the comments or contact us at www.resonance-se.com
The goal of Resonance Simulation and Engineering (RSE) is to be your full-service simulation provider. As our name suggests, our focus is on mechanical vibration analyses. We specialize in modal, harmonic, transient and dynamic finite element analysis (FEA). Additionally, we provide standard simulat...
