Engine cycles and architecture

An engine is an air pump with a fire in it. Almost everything that decides how much torque it makes, how efficient it is and how smoothly it runs comes down to two questions: how much air it can trap in a cylinder, and how much of the heat released in that air it can turn into useful work before the rest goes out of the exhaust pipe or into the coolant.

A sectioned four cylinder petrol engine on a workshop stand, cut lengthways so the four pistons, connecting rods, crankshaft and overhead camshafts are all visible, with the pistons stopped at four different heights in their bores. — The same four parts in four different positions. A sectioned engine shows the whole cycle at once, which is the one thing a running engine never lets you see.

The four strokes, and what each one costs

A diagram of the four stroke cycle. Four cylinder cross sections are shown in sequence, intake with the inlet valve open and the piston descending, compression with both valves shut and the piston rising, power with the spark at top dead centre and the piston driven down, and exhaust with the exhaust valve open and the piston rising. Below them a valve timing circle shows inlet opening before top dead centre and closing after bottom dead centre, exhaust opening before bottom dead centre and closing after top dead centre, with the overlap region near top dead centre shaded. To the right a pressure against volume indicator diagram shows the large clockwise combustion loop that produces work and, drawn much smaller and anticlockwise below it, the pumping loop that consumes work, with the throttled part load case shown as a much larger pumping loop than the wide open throttle case. — One stroke in four makes work. The small anticlockwise loop at the bottom of the indicator diagram is the pumping loss, and throttling makes it bigger.

Intake. The piston descends with the inlet valve open and draws in air, or air and fuel. This is the stroke that sets the ceiling on everything else: whatever mass of air ends up trapped is the most fuel that can be burnt, so every intake, cam, turbocharger and exhaust decision is an attempt to trap more of it.

Compression. Both valves shut and the piston rises, squeezing the charge into a small volume. Compressing costs work, but it is repaid with interest, because a charge that starts hot and dense burns faster and expands through a larger ratio.

Power. Combustion raises the pressure and drives the piston down. This is the only stroke that pays.

Exhaust. The piston rises and pushes the burnt gas out. The heat still in that gas is the single largest loss in the engine — larger than friction, larger than the coolant.

Compression ratio, and the wall it runs into

Compression ratio is the total volume above the piston at bottom dead centre divided by the volume left at top dead centre:

CR = (swept volume + clearance volume) / clearance volume

It is the closest thing an engine has to a single efficiency number. Raise it and the charge expands through a bigger ratio, so more of the heat becomes work. Petrol engines would like to run 14 or 15 to 1. They cannot, because of knock.

This is why a diesel is different in kind, not just in degree. It compresses air alone and injects fuel at the moment combustion is wanted, so there is no premixed charge sitting there waiting to autoignite. Compression ratios of 15 to 18 to 1 are normal, and the limit is set by peak pressure and mechanical strength rather than by knock.

	Petrol, spark ignition	Diesel, compression ignition
Compression ratio	9.5 to 12 naturally aspirated, 9 to 10.5 turbocharged	15 to 18
What is compressed	Air and fuel, in port injection	Air only
Combustion starts	Spark, then a travelling flame front	Autoignition at the injected spray
Load controlled by	Throttling the air, mixture held near stoichiometric	Fuel quantity, air unthrottled
Limited by	Knock	Peak pressure, and soot against nitrogen oxides
Part load efficiency	Poor, because throttling costs pumping work	Good, because there is little to throttle

That last row is the practical one. A petrol engine at light load is working against its own closed throttle, and the anticlockwise pumping loop on the indicator diagram grows large. A diesel does not throttle, which is most of why it uses less fuel in traffic.

Bore, stroke and the shape of the numbers

Swept volume of one cylinder is the bore area multiplied by the stroke:

V = (π/4) × bore² × stroke

Capacity is that figure times the number of cylinders. The ratio of bore to stroke then sets the engine’s character:

Oversquare, bore larger than stroke. More room in the head for large valves, and lower mean piston speed at a given engine speed, so it revs. The chamber is wide and shallow, which is slightly worse for knock and for heat loss.
Undersquare, stroke larger than bore. Longer crank throw gives more leverage, so torque arrives low down. Piston speed limits revs. The chamber is compact, which suits knock resistance and is why most diesels are long stroke.

Mean piston speed, roughly 2 × stroke × rpm, is the figure that actually limits engine speed. Production engines live around 15 to 20 m/s at peak power, and that number has barely moved in decades, because it is set by ring friction and by the inertia loads on the rod.

Cylinder layout, firing order and why smoothness is geometry

A diagram comparing cylinder layouts and their balance. Four end views are shown, an inline four, an inline three, a ninety degree V6 and a flat four, each with its cylinders and crank throws drawn. Beneath each is a summary of which inertia forces and couples cancel and which do not, the inline four marked as having uncancelled secondary forces at twice crank speed, the inline three marked as having a rocking couple, the V6 marked as needing a split crankpin for even firing, and the flat four marked as cancelling primary forces by opposition. To the right, a diagram of an inline four shows all four pistons with arrows for their primary inertia forces cancelling in pairs and their secondary forces all pointing the same way and summing, with a balance shaft drawn below the crankshaft turning at twice engine speed in the opposite direction. Below, a firing order strip shows one two four three against crank angle for an inline four, with the one hundred and eighty degree firing interval marked and a straight six shown beside it with a one hundred and twenty degree interval and overlapping power strokes. — A straight six cancels its own primary and secondary forces and its couples. Everything else is a compromise, managed by counterweights, balance shafts or rubber.

The piston does not move sinusoidally, because the connecting rod swings. Decompose the motion and there is a primary component at crank speed and a secondary component at twice crank speed. Whether those sum or cancel across the cylinders is fixed by the geometry, and no amount of manufacturing quality changes it.

Layout	Primary forces	Secondary forces	Couples	In practice
Inline 4	Cancel	Do not cancel	Cancel	Shakes vertically at 2× rpm; balance shafts above about 2.0 litres
Inline 3	Cancel	Cancel	Rocking couple	Needs a balance shaft or soft mounts; uneven feel is inherent
Inline 6	Cancel	Cancel	Cancel	Inherently balanced, and the reason it is prized
90° V6	Cancel	Partly	Couple present	Needs a 30° split crankpin for even firing
60° V6	Cancel	Partly	Couple present	Even firing without a split pin; narrower
Flat 4	Cancel by opposition	Cancel	Small couple	Smooth, wide, and hard to package
V8, 90°	Cancel	Cancel	Cancel with a cross plane crank	Smooth; the uneven bank firing is what makes the noise

Firing order is a separate matter from balance. Its job is to spread the power strokes evenly around the crank and to avoid loading adjacent main bearings in succession. An inline four fires every 180° of crank rotation, usually 1-3-4-2 or 1-2-4-3, so there is a gap between power strokes. A straight six fires every 120°, and because a power stroke lasts 180° the strokes overlap — there is always at least one cylinder pushing. That overlap, not just the force cancellation, is why a six feels different.

The two stroke, and the rotary

The two stroke completes its cycle in one revolution, so in principle it makes twice the power for the same capacity and speed. The catch is scavenging: fresh charge and exhaust have to share the cylinder during the same brief window near bottom dead centre, so they mix at the boundary and some fresh fuel goes straight out. In a crankcase scavenged engine the oil is burnt with the fuel. It is a good cycle for a chainsaw and an impossible one for a modern emissions limit.

The Wankel rotary has excellent power density and almost no reciprocating imbalance, but its combustion chamber is long and thin with a high surface to volume ratio, so heat loss and hydrocarbon emissions are high, and the apex seals live a hard life. Its remaining role is as a range extender turning a generator at one fixed speed, where those weaknesses matter least.

Reading a spec sheet honestly

Two terms separate what happened in the cylinder from what arrived at the shaft:

Indicated work is what the gas did on the piston — the area of the combustion loop on the indicator diagram.
Brake work is what is left at the output shaft after friction and pumping losses, measured on a dynamometer. SAE J1349 and ISO 1585 exist to fix the correction factors and accessory load so that two published figures can be compared at all.

BMEP, brake mean effective pressure, is brake torque normalised by capacity, and it is the honest way to compare engines of different sizes. A naturally aspirated petrol engine sits around 10 to 12 bar BMEP; a modern turbocharged one reaches 20 to 25 bar. BSFC, brake specific fuel consumption in grams per kilowatt hour, is the efficiency number, best around 200 to 210 g/kWh for a good diesel and 230 to 250 for petrol.

And the relationship that governs everything downstream of the flywheel:

Power = torque × angular speed

Torque is what the engine achieves per combustion event. Revs are how often it gets one. This is why two engines with identical peak torque can be three times apart in power, and why the answer to almost every mismatch between them is a gearbox.

What to take away

An engine is an air pump; trapped air mass sets the ceiling on everything else.
One stroke in four makes work, and the camshaft turns at half crank speed.
Compression ratio is the efficiency lever, and knock is the wall it hits.
A diesel escapes that wall by compressing air alone and injecting at the moment of combustion.
A petrol engine’s part load penalty is pumping work against its own throttle.
Oversquare revs, undersquare pulls; mean piston speed is the real rev limit.
An inline four cannot cancel its secondary forces; a straight six cancels everything.
Firing order spreads the power strokes and protects the bearings — it is not balance.
The two stroke loses on scavenging, the rotary on surface to volume ratio.
BMEP compares engines of different capacity; BSFC is the efficiency figure.