How a four-stroke engine works

Kiran Dhaliwal has been at Ridgeline Auto Service for four months. She can now do an oil service, a tire swap and a multi-point inspection without anyone checking every step, and Marty Kowalchuk has decided it is time she learned what happens inside the engines she has been servicing. On a Monday morning he rolls an engine stand into bay 5. On it sits a 2.0 litre four-cylinder engine from a car that was written off after a collision, which Ridgeline keeps for training. The cylinder head is off, the oil pan is off, and the pistons can be seen from above and below.

“Everything you do on this engine for the next few weeks you will do for real on a customer's car later,” Marty says. “Measure it, take it apart, put it back together. But first you need to know what it does.” He turns the crankshaft slowly with a breaker bar on the front bolt, and Kiran watches two pistons rise while the other two fall.

This lesson explains the basic job of a piston engine: turning the chemical energy in fuel into the rotation that drives the wheels. It covers the main moving parts, the four strokes of the cycle, why an engine needs two crankshaft turns to complete one cycle, how the cylinders take turns firing, and the variations used in hybrid and diesel engines. Every later lesson in this course, from valve trains to diagnosis, builds on this one.

From fuel to rotation

A gasoline engine is a heat engine. It burns a mixture of fuel and air inside a closed space, the combustion chamber, and the hot gas pushes on a piston. The piston is connected by a connecting rod to the crankshaft, and the offset journal on the crankshaft, the crankpin, turns the piston's straight-line motion into rotation, the same way a cyclist's legs turn the pedals. Because the burning happens inside the engine itself, it is called an internal combustion engine.

Only part of the fuel's energy becomes useful work at the crankshaft. In a typical modern gasoline engine about a third of it does, at best, and less at light loads. The rest leaves as heat in the exhaust gas, as heat carried away by the coolant and oil, and as friction. That is why an engine needs a cooling system and a lubrication system, which Lessons 8 and 9 cover, and why small improvements in friction or combustion are worth so much to manufacturers.

Top dead centre, bottom dead centre and the stroke

As the crankshaft turns, the piston moves between two end points. At top dead centre, or TDC, the piston is at the top of its travel and the space above it is smallest. At bottom dead centre, BDC, it is at the bottom and the space above it is largest. The distance between them is the stroke, and it equals twice the crankpin's offset from the crankshaft's centre, the crank throw. The diameter of the cylinder is the bore. Lesson 3 uses bore and stroke to calculate an engine's displacement.

Kiran notices that the piston slows almost to a stop near TDC and BDC and moves fastest about halfway between them. Marty explains that this is why a piston's speed is quoted as an average, the mean piston speed, and why the top piston ring wears the cylinder most at the very top of the stroke, where the piston stops and reverses with full combustion pressure behind it. She will measure that wear on the training engine in Lesson 7.

The four strokes

Almost every car and light truck engine works on the four-stroke cycle, also called the Otto cycle after Nikolaus Otto, whose engine of 1876 first used it successfully. One complete cycle takes four strokes of the piston, which is two turns of the crankshaft, or 720 degrees.

• Intake stroke. The piston moves down from TDC to BDC with the intake valve open. The falling piston lowers the pressure in the cylinder, and the higher atmospheric pressure outside pushes air, or air and fuel, in through the intake port.
• Compression stroke. Both valves are closed and the piston moves up, squeezing the mixture into the small space of the combustion chamber. Compressing the mixture raises its pressure and temperature, which makes it burn faster and lets the expanding gas do more work.
• Power stroke. Near the top of the compression stroke the spark plug fires. The burning mixture's pressure rises sharply and drives the piston down. This is the only stroke that delivers work to the crankshaft; the others take work from it.
• Exhaust stroke. Near the bottom of the power stroke the exhaust valve opens. The piston moves up and pushes the burned gas out through the exhaust port, and the cycle begins again.

In a real engine the valves do not open and close exactly at TDC and BDC. The intake valve opens a little before TDC and closes well after BDC, so the moving air keeps filling the cylinder even as the piston starts back up. The exhaust valve opens before BDC to let the pressure blow down, and closes after TDC. For a short time near TDC at the end of the exhaust stroke both valves are slightly open; this is called valve overlap. The camshaft's shape sets these timings, and variable valve timing, in Lesson 6, changes them while the engine runs.

The spark also comes before TDC. The mixture takes a few thousandths of a second to burn, so the engine control module fires the plug early enough that the peak pressure arrives just after TDC, when it can push the piston down most effectively. The amount of spark advance increases with engine speed and falls with load. Ignition is covered in AUTO-230; in this course it is enough to know that the spark, the valves and the piston all have to be in step, and that the timing chain or belt is what keeps the camshaft and crankshaft in step.

Firing order and firing interval

An engine with several cylinders arranges their power strokes so that they are spread evenly over the 720 degrees of the cycle. In a four-cylinder engine, a power stroke starts every 180 degrees of crankshaft rotation; in a V6, every 120 degrees; in a V8, every 90 degrees. The sequence in which the cylinders fire is the firing order, and it is cast into the intake manifold or printed in the service information. A common four-cylinder firing order is 1-3-4-2. Gord Haney's 5.3 litre V8 fires 1-8-7-2-6-5-4-3, the order used by this family of General Motors V8 engines.

Cylinder numbering is set by the manufacturer and must always be checked. On many V engines the odd-numbered cylinders are on one bank and the even-numbered ones on the other; on others the cylinders on one bank are numbered 1 to 3 and on the other 4 to 6. A technician who tests or replaces parts on the wrong cylinder because of a guess wastes the customer's money, so Marty's rule from AUTO-110 applies: look it up.

In this formula, theta is the crankshaft angle between power strokes and n is the number of cylinders. The more cylinders an engine has, the more often a power stroke occurs and the smoother it runs.

# Firing interval and how long each stroke lasts at a given engine speed
for name, cylinders in [("four-cylinder", 4), ("V6", 6), ("V8", 8)]:
    interval = 720 / cylinders
    print(f"{name:<14} power stroke every {interval:5.1f} degrees")
print()
for rpm in (700, 2000, 6000):
    rev_per_s = rpm / 60
    ms_per_stroke = 1000 / rev_per_s / 2          # one stroke is half a revolution
    power_per_s_v8 = rev_per_s / 2 * 8            # each cylinder fires once every 2 revolutions
    print(f"{rpm:>5} rpm: one stroke takes {ms_per_stroke:5.1f} ms; a V8 makes {power_per_s_v8:4.0f} power strokes per second")

four-cylinder  power stroke every 180.0 degrees
V6             power stroke every 120.0 degrees
V8             power stroke every  90.0 degrees

  700 rpm: one stroke takes  42.9 ms; a V8 makes   47 power strokes per second
 2000 rpm: one stroke takes  15.0 ms; a V8 makes  133 power strokes per second
 6000 rpm: one stroke takes   5.0 ms; a V8 makes  400 power strokes per second

At 6,000 rpm each stroke lasts five thousandths of a second, which is why the intake air must be helped by the timing of the valves and why the parts inside an engine must be balanced and lubricated so carefully. At idle, a V8 still makes nearly fifty power strokes every second.

Following one cylinder through the cycle

Marty asks Kiran to find TDC on cylinder 1 of the training engine. She puts a long screwdriver gently on the top of the piston, turns the crankshaft clockwise by hand until the screwdriver rises to its highest point, and checks that the timing mark on the crankshaft pulley lines up with the zero mark on the front cover. Then he asks her which stroke cylinder 1 is on. She cannot tell, because the head is off and there are no valves.

“That's the point,” he says. “The crankshaft passes TDC twice in each cycle, once at the end of compression and once at the end of exhaust. The crank looks the same both times. Only the camshaft knows which one it is.” On an assembled engine, TDC on the compression stroke is the position where both valves of that cylinder are closed and their rockers are loose; on the exhaust stroke, the valves are rocking, with the exhaust closing and the intake just opening. Finding TDC compression on cylinder 1 is the starting point for setting valve lash, for timing a camshaft and for many timing-chain jobs, and she will do it again in Lessons 4, 5 and 12.

What makes the engine run: air, fuel, compression and spark

A gasoline engine needs four things in each cylinder: enough air, the right amount of fuel mixed with it, compression, and a spark at the right time. Technicians use this list as a starting point when an engine will not start or a cylinder does not fire. A cylinder that has fuel and spark but cannot hold compression, because a valve is bent, a ring is broken or a head gasket has failed, will not fire properly. A cylinder whose valves do not open, because a camshaft lobe is worn or a lifter has collapsed, cannot breathe and will not fire either. Lesson 10 shows how compression, leak-down and vacuum tests tell these faults apart.

The amount of air an engine takes in sets how much fuel it can burn and therefore how much power it can make. Opening the throttle lets more air in. A turbocharger or supercharger pushes air in above atmospheric pressure. At high altitude, where the air is thinner, a naturally aspirated engine takes in less air and makes less power. Calgary sits at about 1,045 m above sea level, and Lesson 3 calculates how much power an engine loses there.

Two strokes, diesels and hybrid engines

Some small engines, such as those in chainsaws, outboard motors and older snowmobiles, use a two-stroke cycle, which completes intake, compression, power and exhaust in one crankshaft turn by using ports in the cylinder wall instead of valves. They are light and powerful for their size but use more fuel and produce more emissions, and are no longer used in cars.

Diesel engines also use the four-stroke cycle, but they have no spark plugs. They compress air alone to a much higher ratio, roughly 15 to 20 to one compared with 9 to 12 to one in a gasoline engine, and the air becomes hot enough to ignite the fuel when it is injected near the top of the compression stroke. This is called compression ignition. Diesels are more efficient, especially under load, and are used in heavy pickups, many vans and nearly all large trucks.

Ines Moreau's hybrid hatchback uses a gasoline engine that runs on the Atkinson cycle, a version of the four-stroke cycle in which the intake valve stays open well into the compression stroke. Some of the mixture is pushed back into the intake, so the effective compression stroke is shorter than the expansion stroke. The engine makes less power for its size but uses its fuel more efficiently, and the hybrid's electric motor supplies the extra power when it is needed. The engine also stops and starts many times on every trip, which affects how it is lubricated and cooled. Dr. Amara Singh's electric car has no engine at all, and Kiran will meet its electric drive in AUTO-410.

Rotation, direction and turning an engine by hand

Most engines turn clockwise when seen from the front, the timing chain end. Service procedures that ask a technician to turn the engine by hand assume this direction, and turning an engine backwards can let a timing chain tensioner retract or a belt jump a tooth. The crankshaft is turned with a socket on the front bolt, never by the camshaft sprockets, and always slowly, so that if something stops it, such as a valve touching a piston, the technician feels it before damage is done.

Before turning an engine by hand in a vehicle, Kiran learns to disable the ignition and fuel, for example by removing the fuel pump relay or the ignition fuse as the service information directs, so the engine cannot start if someone turns the key. It is also easier to turn an engine with the spark plugs removed, because the cylinders are no longer compressing air.