A gasoline engine uses spark ignition to release energy from a fuel-air mixture inside a cylinder. Gas pressure moves a piston connected to a crankshaft. Explore the four-stroke motion here and compare a power range obtained by multiplying displacement by two power-density values of your choice.
Gasoline Engine Interactive Calculator
Explore a user-defined power-density range by multiplying engine displacement by entered low and high bhp-per-litre assumptions, alongside exact ideal four-stroke slider-crank kinematics.
Equation Used
- The user supplies both power-density endpoints.
- The endpoints are reordered if entered low exceeds entered high.
- The midpoint is only the arithmetic mean of the resulting endpoints.
- No performance, combustion, pressure, speed, thermal, structural, emissions or safety model.
Scenario arithmetic only. The entered bhp/L values are assumptions, not predictions or safe targets. The calculator does not model speed, torque, cylinder pressure, breathing, combustion, efficiency, heat, emissions, loads, strength or durability.
Four strokes and two crank revolutions
The intake stroke increases cylinder volume to draw in a charge. Compression reduces that volume. Combustion raises the pressure, and expansion during the power stroke drives the piston. The exhaust stroke expels residual gas. A complete four-stroke cycle takes two crankshaft revolutions.
The piston and connecting rod in the animation follow an ideal planar slider-crank. The cycle runs at a fixed, deliberately slow speed for viewing. It does not calculate gas pressure, valve timing or shaft power, and changing displacement does not change the schematic dimensions.
Comparing power-density assumptions
Power per litre is a convenient way to express a measured engine output relative to displacement. Here, both endpoint values are supplied by the reader. Multiplying them by displacement shows the corresponding power interval.
The resulting interval can help organize a comparison when the assumptions come from relevant measurements. It cannot establish whether an engine can achieve those values or how its output changes with speed.
Displacement times power density
For displacement V in litres and endpoint values a and b in bhp/L, the lower output is V × min(a,b) and the upper output is V × max(a,b). Mid Power is their arithmetic mean; Power Span is upper minus lower.
The result uses brake horsepower as the entered unit of shaft power. No pressure, speed or efficiency appears in the equation. It therefore scales assumptions rather than predicting brake power from an engine design.
Example: scaling an entered interval
With 1.0 L displacement and endpoints of 50 and 150 bhp/L, the results are 50 and 150 bhp. Mid Power is 100 bhp and Power Span is 100 bhp. At 2.0 L with those same assumptions, all four results double.
If the first endpoint is 150 and the second 80 bhp/L, the calculator sorts them: at 1.0 L, the displayed lower and upper outputs are 80 and 150 bhp, with midpoint 115 bhp and span 70 bhp. The entry labels remain in their original order, so check the sorted result tiles.
Across the controls, results remain finite, the largest possible upper endpoint is 2000 bhp and the smallest lower endpoint is 10 bhp. Equal entered endpoints produce zero span. These are selectable arithmetic limits, not engine ratings.
Scope and limitations
Power density is a compact comparison quantity, but displacement alone cannot predict output. A real analysis requires an identified engine, speed and torque or measured pressure data, intake and exhaust conditions, fuel, ignition, efficiency, thermal limits, emissions constraints, component loads and manufacturer limits.
Frequently asked questions
Are the default power-density values recommendations?
No. They are editable assumptions. Use relevant engine data if you want to compare measured examples.
What does Mid Power mean?
It is the arithmetic mean of the lower and upper outputs, not a predicted or most likely engine rating.
What happens if the endpoints are entered in reverse order?
The outputs are sorted numerically before the midpoint and span are calculated.
Why does changing displacement not change the piston drawing?
The schematic uses fixed proportions to show the motion. Displacement only scales the power-density calculation.
Can this predict output or guide engine tuning?
No. It has no combustion, breathing, speed, thermal or durability model.
Technical context
Building or designing a mechanism like this?
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