The modern internal-combustion engine is – at its heart – an intricate balance of specific events. Intake, enrichment, compression, ignition and exhaust all act in harmony to efficiently produce power. When these events all happen in the correct order – and in the right balance – the engine will run smoothly.
Even minor inconsistencies can rob an engine of its power-producing potential. Aspiring engine-performance technicians enrolled in an accredited automotive repair school come to understand this balance – and those factors which can negatively affect it.
The Combustion Cycle
In a traditional engine, the combustion cycle incorporates the events outlined below. Each cylinder in the engine will perform a different event in this cycle at a different point within the crankshaft rotation.
- Intake Stroke: With the intake valves open, the piston moves down within the bore to draw oxygen-rich air into the cylinder and combustion chamber.
- Enrichment: As the cylinder and combustion chamber fill with air, the fuel injector fires – adding atomized gasoline to make the air flammable.
- Compression Stroke: As the intake valves close, the piston moves up within the bore to compress the air/fuel mixture into the combustion chamber.
- Ignition: With the air/fuel mixture compressed to a high density; the spark plug fires – igniting the oxygen-rich contents of the combustion chamber.
- Power Stroke: With valves closed, combustion forces the piston down in the bore – turning the crankshaft and moving other pistons into their event stages.
- Exhaust Stroke: With the exhaust valves open, the piston moves up within the bore to force spent gases out of the cylinder and into the exhaust stream.
Factors that Influence Performance
Airflow into an engine is the primary influence on performance. Minimizing restriction allows an engine to run efficiently and make more power. Restriction can slow airflow and create turbulence. Timing controls the opening and closing of valves relevant to crankshaft rotation and piston position. Proper timing allows efficient filling of chambers and scavenging of spent gases.
Compression tells us the ratio to which the air/fuel mixture is compressed at the height of compression. At higher ratios, more oxygen is present. A dense mixture produces more power when ignited. Like the intake, exhaust performs best with minimal restriction. Effective exhaust relies on minimal back pressure and effective scavenging of spent gases before the intake stroke.
Understanding Octane Ratings and Performance
A common misconception is that high-octane gasoline produces more power. The truth is different. The higher the octane, the harder it is to ignite the fuel. When compressed, the air/fuel mixture is easily ignited under pressure. If it lights during compression but before ignition, the force works against crankshaft rotation. This is “detonation” or “knock.” High octane allows higher compression without risk of detonation. That is why high-performance engines specify higher-octane fuel. Low-compression engines, however, run less efficiently and produce less power on high-octane gasoline.
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