Otto Engines: Power

A. Smith University of Utah Department of Mechanical Engineering

The Otto cycle is a model for how a typical internal combustion spark ignition (SI) engine works. The ideal Otto cycle is a serious of four processes: adiabatic compression, constant-volume heat addition, adiabatic expansion, and constant-volume heat rejection. In this model, the piston would cover the stroke distance twice (hence 2-stroke) and the crankshaft would rotate once for every modeled thermodynamic cycle (meaning the series of state changes from 1→2→3→4→1 which result in a net work output). 2 strokes → 1 rev →1 cycle

In reality, an SI engine is usually a four-stroke series of processes where the piston covers the stroke distance four times (hence 4-stroke) and the crankshaft rotates twice for every “thermodynamic cycle” (remember that a real engine’s working fluid does not experience a full thermodynamic cycle—it doesn’t get back to the state where it started). This is a more efficient and environmentally friendly way to produce power than a real-life 2-stroke cycle. 4 strokes→ 2 rev → 1 cycle

With multiple cylinders, the position and valve timing of each cylinder is designed in such a way that the individual power strokes are offset, so the crankshaft is experiencing a power stroke every half revolution or more often (4 or more cylinders). Remember that power output is the rate of delivering work: W ork  Cycle P ower = Cycle time

Say you have a simple 2-stroke engine operating at 1000 RPM, and the work produced by a cycle is given in J. The power produced by this 2-stroke, single cylinder engine, in Watts, is: W ork[J] 1000rev  1cycle min P ower[W ] = cycle min 1rev 60s

Say your 4-cylinder car is cruising along at 2500 RPM, and the work produced by an individual cylinder is given in lbf-ft. The power produced by this 4-stroke, 4 cylinder engine, in horsepower, is:           W ork[lbf − ft] 2500rev 1cycle min hp P ower[hp] = 4∗ cycle min 2rev 60s 550lbf − ft/s

A number of animations can be found online which illustrate the stages of 4-stroke Otto Cycle power production. Follow what the cylinder is doing when the intake, compression, combustion, expansion, and exhaust processes take place. Which stroke is the power stroke?