Engine Fundamentals

To fix an engine, you need a clear picture of what it is supposed to be doing. Every engine needs the same things to run: air, fuel, spark, compression and correct timing. When something goes wrong, you ask which of those is missing. This guide explains how a four-stroke gasoline engine works and how its supporting systems keep it alive.

What You Will Be Able to Do

• Describe the four strokes and how far the crankshaft turns to complete them
• Name the main engine parts and what each one does
• Explain why valve timing matters and what a timing belt or chain does
• Tell port injection from direct injection and explain the trade-off
• Explain what oil, coolant and the PCV system do for the engine

The Main Parts

The top of a piston's travel is called Top Dead Center, or TDC. The bottom is Bottom Dead Center, or BDC.

The Four-Stroke Cycle

Each cylinder repeats four strokes. Together they take two full turns of the crankshaft, or 720 degrees.

1. Intake. The piston moves down and the intake valve is open. Air, or air and fuel on a port-injected engine, fills the cylinder.
2. Compression. Both valves close and the piston moves up, squeezing the mixture. A typical gasoline engine has a compression ratio somewhere around 9:1 to 13:1.
3. Power. The spark plug fires shortly before TDC. The burning mixture pushes the piston down. This is the only stroke that produces power.
4. Exhaust. The exhaust valve opens and the piston moves up, pushing burned gas out.

The camshaft turns at half the crankshaft's speed, because each valve opens once for every two turns of the crank.

If any stroke fails, the cylinder stops contributing. A leaking valve loses compression. A dead coil loses spark. A clogged injector loses fuel. That is why diagnosis comes back to the same few basics.

Valve Timing

The valves do not open exactly at TDC and BDC. The intake valve opens a little before TDC, and the exhaust valve closes a little after it. The short time when both are open is called overlap, and it helps clear out exhaust gas.

Most modern engines use variable valve timing, or VVT. Oil pressure, directed by a solenoid, moves a phaser on the camshaft to change when the valves open as speed and load change. Some engines also change how far the valves lift. VVT depends on clean oil of the correct grade, so sludge, low oil or the wrong viscosity can set cam timing codes, commonly in the P0010 to P0024 range.

Valve Lash

Valve lash, or valve clearance, is a small gap in the valve train between the camshaft and the valve. It allows for parts growing as they heat up. Too little gap can hold a valve open and burn it. Too much makes noise and reduces valve opening. Many engines use hydraulic lifters that take up the gap on their own. Others need checking and adjusting at set intervals.

Timing Belts and Chains

A timing belt is rubber and has a replacement interval. A timing chain runs in oil and is meant to last, though it can stretch.

Engines are either interference or non-interference. In an interference engine, the valves and pistons share the same space at different times. If the belt or chain breaks, the pistons strike the open valves, which bends them and can do severe damage. Many modern engines are interference designs, so a missed timing belt can destroy an engine.

Engine Layouts

On a V engine, each side is called a bank. Bank 1 is always the bank with cylinder 1.

The firing order is the sequence in which the cylinders fire, spaced to keep the engine smooth. Many inline-4s fire 1-3-4-2, and GM small-block V8s fire 1-8-7-2-6-5-4-3. Cylinder numbering differs between makers, so check the service information before you act on a cylinder-specific code.

Air and Fuel

The engine computer measures incoming air, using a mass air flow or manifold pressure sensor, and calculates how much fuel to inject. Oxygen sensors in the exhaust report back so the computer can correct the mixture.

On a GDI engine, fuel goes straight into the cylinder, so it never washes over the back of the intake valves. Oil vapor from the crankcase bakes onto them as carbon. Over time this causes rough idle and misfires. The usual fix is a mechanical cleaning, often walnut shell blasting.

Warning: Fuel is flammable, and a GDI system holds very high pressure. Never open a fuel line until you have followed the service procedure to relieve pressure. High-pressure fuel can pierce skin.

Ignition

Most modern engines have one coil per cylinder, mounted on the spark plug. This is called coil-on-plug. The coil turns 12 volts into tens of thousands of volts, enough to jump the plug gap.

Warning: Ignition systems can give a painful shock that makes you jerk into moving parts. It is more serious if you have a pacemaker or heart condition. Do not handle coils or plug wires with the engine running.

Spark plugs differ in material and life. Copper plugs wear fastest, and platinum and iridium plugs last much longer. Check the gap, and tighten to the torque spec. Plug makers generally advise against anti-seize on plated plugs because it changes the torque.

Lubrication

Engine oil's main job is to lubricate moving parts and reduce friction. It also carries away heat, cleans, seals the rings and feeds hydraulic parts like lifters and VVT phasers.

The oil pump draws oil from the pan through a pickup screen and pushes it through a filter to the bearings and valve train. The bearings ride on a thin film of pressurized oil, not on metal.

An oil grade such as 0W-20 has two parts. The number before the W, for winter, describes how easily the oil flows when cold at startup. The second number describes its thickness at operating temperature. The lower the first number, the better the cold flow. Use the grade the manufacturer specifies, since the wrong one can affect VVT operation, oil use and warranty coverage.

Warning: Oil pressure warning on? Shut the engine off now. Running with low oil pressure can destroy bearings in minutes.

Typical warm oil pressure is low at idle and rises with rpm, but the spec varies by engine. Causes of low pressure include low oil level, the wrong viscosity, a clogged pickup screen, a worn pump and worn bearings.

Cooling

Combustion produces far more heat than the engine can use. Coolant carries that heat from the block and head to the radiator.

The water pump circulates coolant. The thermostat blocks flow to the radiator until the engine reaches operating temperature, then opens to let it circulate, which warms the engine quickly and holds it steady. A pressure cap raises the boiling point. A 50/50 coolant mix with a 15 psi cap boils at about 265°F, so the system can run hotter than water alone.

Overheating can warp a cylinder head and blow a head gasket. Running too cold wastes fuel and increases wear.

Crankcase Ventilation

A little combustion gas always gets past the rings into the crankcase. This is called blow-by. The Positive Crankcase Ventilation, or PCV, system routes those gases from the crankcase back into the intake to be burned. That stops pressure building up, reduces emissions and keeps out moisture and sludge. A stuck PCV valve can cause oil leaks, oil use or a vacuum leak.

What Can Go Wrong

Use this as a first map of symptoms, then test before you decide.

Common Mistakes

Check Your Understanding

1. A cylinder has spark, fuel and good timing but will not fire. Which of the basics do you check next?
2. A customer's timing belt snapped at highway speed. The engine is an interference design. Why does this matter?
3. An engine has a P0014 cam timing code, and the oil is black and low. Why might the oil be connected?
4. A direct-injected engine with 90,000 miles has a rough idle. Why might the intake valves be part of the problem?

Answers

1. Compression. Valves, rings or a head gasket could be letting it escape.
2. The pistons have likely hit open valves, so the damage is probably far beyond a new belt.
3. VVT phasers run on oil pressure, so low or dirty oil can stop them working correctly.
4. Direct injection does not wash the valves with fuel, so carbon can build up on them and disturb airflow.

Key Takeaways

• An engine needs air, fuel, spark, compression and correct timing.
• One four-stroke cycle takes two crankshaft turns, or 720 degrees.
• Interference engines can suffer severe damage if a belt or chain breaks.
• Direct injection improves efficiency but allows carbon on intake valves.
• Oil lubricates, cools and cleans. Stop the engine on a low oil pressure warning.
• The thermostat holds the engine at operating temperature, and the PCV system recycles blow-by gases.