How does a piston turn fuel into motion?

Fill and squeeze

On the intake stroke, the descending piston draws the fuel–air mixture through the open intake valve. During compression, both valves close and the rising piston reduces the occupied volume.

Push and clear

Near the end of compression, a spark starts combustion. The expanding gas does work on the descending piston. The next upward stroke pushes exhaust out through the exhaust valve. The flywheel stores rotational energy across the cycle.

A valve train keeps time

This overhead-valve teaching assembly shows a camshaft, followers, pushrods and rockers. A 2:1 timing drive makes the cam turn once for two crank turns. Rockers transmit the pushrod motion to valves in the head. Small engines used in mowers provide a familiar application.

From a whole engine to its assembly

The cutaway removes the front of the cylinder wall to expose the piston. In the exploded view, the head, piston, rod, flywheel and cover separate along inspection directions. Return to the assembled or cutaway view before interpreting connected motion; separated parts are not operating positions.

Slider-crank geometry

The crank pin follows a circle, but the piston must remain on a straight guide. With crank radius r and rod length l, downward travel from the top is r+l−r cos θ−√(l²−r² sin² θ). Here r=30 mm and l=120 mm. At 90°, travel is about 33.81 mm, slightly more than halfway down.

Same piston position, different state

At 0° and 360° the piston is at the top. At 0° intake is about to begin; at 360° expansion is about to begin. The complete ideal sequence repeats at 720°. The study diagram separates crank angle from stroke identity.

Motion is not an engine test

You supply the crank motion in this model. Removing the spark removes the combustion cue while the externally turned mechanism still moves. Actual speed, torque, pressure, fuel consumption and stalling require combustion and rotational-dynamics models.

The cam, follower and rocker agree

The visible cam surface is derived from a radial roller-follower pitch curve. Its prescribed lift rises and returns smoothly during one ideal stroke. A fixed-length pushrod and rocker determine the valve lift. Roller size, cam lift and link dimensions are teaching design choices, not measurements from a commercial engine.

A mower, generator or water pump

A small four-stroke engine can supply rotating mechanical power to different loads. The blade, generator or pump does a different job downstream; the piston, connecting rod and crankshaft still provide the same basic conversion.

Why other engines look different

A single-cylinder engine makes the whole cycle easy to inspect. Multiple-cylinder arrangements place power events at different crank angles. Two-stroke and compression-ignition engines require different explanations; this model does not substitute for them.

The piston is back at the top after one crank turn. What has repeated?

• Both piston position and the entire four-stroke cycle
• Piston position, but not the entire cycle
• Neither piston position nor the cycle

You pause halfway through compression. Which valves should be open in this ideal model?

• Intake only
• Exhaust only
• Neither valve

The crank gear has 20 teeth and the cam gear has 40. Two crank turns make the cam turn…

• Once, in the opposite direction
• Twice, in the same direction
• Four times, in the opposite direction

Why does the connecting rod tilt during the cycle?

• Its length changes to reach the piston
• One end follows a circle while the other follows a straight line
• The cylinder tilts with it

After the cam’s raised part passes, what closes the valve in this assembly?

• The valve spring
• The spark plug
• The piston pulls it closed

Where does the useful energy entering this gasoline engine mainly come from?

• The spark’s tiny electrical energy alone
• Chemical energy transferred during fuel combustion
• Energy created by the crankshaft

What does a flywheel help a running engine do between power strokes?

• Store and release rotational kinetic energy
• Create another power stroke
• Keep both valves open

You omit the spark, but this model still turns. What can you conclude?

• Real engines never need a spark
• The displayed motion is supplied externally; stalling is not modeled
• The flywheel supplies unlimited energy

A generator and a water pump both use the same kind of four-stroke engine. What changes first in your explanation?

• The four strokes disappear in the pump
• The downstream load and purpose of the shaft’s rotation
• The cam must rotate twice as fast as the crank

A complete four-stroke cycle takes two crankshaft turns.

Manufacturer explanation of intake, compression, power, exhaust and flywheel role.

An overhead-valve engine uses a timed cam, pushrods and rockers.

Original manufacturer video. Reference for architecture, not copied footage or model geometry.

Expansion can deliver work; an ideal cycle differs from actual engine operation.

Technical education on ideal compression, heat input and expansion. No efficiency or horsepower prediction is made here.

A real small OHV engine connects the crank, cam, tappets, pushrods, rockers and spring-return valves.

Manufacturer exploded assembly reference for the relationship of parts. Original teaching geometry is used; no Honda diagram or CAD is redistributed.