Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?


Gasoline Engine Basics

Gasoline engines convert fuel and air into torque. Aside from the fuel and air/exhaust systems, ignition, cooling, and lubrication systems are also needed. Sensors are used to meter the appropriate amount of fuel through the injectors, unless you have a carbureted engine.

Fuel system

The fuel pump, which is usually inside the fuel tank, pumps fuel to the fuel rail on top on the engine. Normal fuel pressure is 30-60 psi, and is usually regulated using a vacuum-operated diaphragm. Excess pressure is relieved and routed back to the tank. The fuel filter may be anywhere between the tank and the engine. Fuel is metered through the injectors by the computer, using inputs from the throttle position sensor, mass airflow sensor, crankshaft sensor, upstream oxygen sensor(s), and possibly the intake manifold absolute pressure sensor. Some cars have a fuel valve that closes in case of an accident. The valve may use a signal from the air bag circuit or may use an integral accelerometer to sense the crash. These valves must be reset manually to restore fuel flow.

Air/exhaust system

Air is metered by the throttle as it is drawn into the intake manifold by the pistons that are on their intake stroke. A camshaft is used to actuate combustion chamber intake and exhaust valves. The intake valves are opened to allow the air/fuel mixture into the chamber and are then closed for the compression stroke. The spark plug ignites the mixture, which burns to expand the combustion products during the power stroke. The exhaust valves are then opened to allow the exhaust stroke to push the combustion products out the exhaust manifold(s), through the catalytic converter, through the muffler, and out the tailpipe.

Ignition system

The ignition system delivers voltage to the coils and interrupts that voltage to produce sparks and deliver them to the spark plugs when the computer determines is the appropriate time. The computer uses inputs from the crankshaft position sensor and possibly knock sensors to establish optimal spark timing. Camshaft position sensors may also be used, if available.

Cooling system

The cooling system pumps coolant through jackets in the engine block and heads to cool the combustion chamber. The hot coolant is routed through the radiator for cooling and then returned to the pump. The coolant is cooled in the radiator using airflow from the vehicle speed and fan(s). A thermostat is used to control the coolant flow rate to maintain coolant temperature. Coolant is also routed through the heater core. Most designs allow this flow to occur even when the thermostat is closed, so that the heater works as soon as the engine begins to heat. Most engines need about 15 minutes to reach normal operating temperature, depending on the local weather. An additional valve is sometimes used in the heater core loop to shut off flow to the heater when the control is set to air conditioning or vent.

Lubrication system

The lubrication system pumps oil at 30-60 psi from the oil sump to the journal bearings in the crankshaft, camshaft(s), and other areas that require lubrication. Oil pressure may also be used to maintain timing chain tension and valve train clearances as well as to adjust valve timing in equipped engines. An oil cooler may be added in high performance engines.


Emission control Systems

Exhaust gas recirculation

A hot engine may produce environmentally harmful oxides of nitrogen that would be released to the atmosphere. Recirculation of some exhaust products back to the intake can reduce the production of these oxides. A valve is used to control the release of exhaust products from the exhaust manifold to the intake manifold. The computer uses inputs from the temperature sensor and crankshaft sensor and possibly the manifold pressure sensor to control the valve.

Evaporation/hydrocarbon control

Most states restrict the release of gasoline vapors from automotive fuel systems, engines, and tailpipes. For this reason, tank caps have been sealed now for decades. To prevent pressure buildup in the tank, a port near the top of the tank allows vapors to travel into a charcoal canister, where they are stored as liquid. The engine computer uses inputs from the crankshaft sensor to open a purge valve to the intake manifold. The valve allows a small flow of vapor from the canister into the engine for burning, thus purging the canister while driving. A separate vent valve may be used to allow vapors to control flow from the tank to the canister. A catalytic converter is usually used to reduce the flow of uncombusted fuel out of the tailpipe. A downstream oxygen sensor is often used to verify the efficiency of the converter in reducing hydrocarbons to lawful levels. A positive crankcase ventilation circuit is used to draw a slow flow of air from the crankcase into the intake manifold to prevent combustible gases that get past the compression rings from being released to the atmosphere.


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