When you want to increase engine power on a four-stroke engine, it is the efficiency of each stroke, particularly the intake and exhaust strokes that you need to improve. Understanding the four-stroke cycle of the internal combustion engine and how it produces power is important when you want to increase engine power. So let's begin with the four-stroke cycle which is also known as the Otto cycle. If you're familiar with the four strokes of the Otto cycle feel free to head on over to power basics. Otherwise read on as this section is important to understanding engine tuning.
The Intake Stroke
The intake stroke is the first stroke of the Otto cycle. During this stroke the intake valve opens as the piston moves from top dead center (TDC) to bottom dead center (BDC). The downward movement of the piston creates a vacuum in the cylinder that causes air/fuel mixture to be drawn through the intake system and into the cylinder. The intake valve usually opens slightly before the stroke begins and closes slightly after the stroke ends to maximize the amount of air/fuel mixture that can be drawn into the cylinder.
The volume of air/fuel mixture that is drawn into the cylinder, relative to the volume of the cylinder, is called the Volumetric Efficiency (VE) of the engine. Maximizing the VE of the engine is an effective method of engine tuning that we can use to increase engine power, especially as stock engines generally have a VE in the range of 85% while older engines have a VE in the range of 70%.
The Compression Stroke
The compression stroke is the second stroke of the Otto cycle. During this stroke both the intake and exhaust valves are closed. The piston moves from BDC back up to TDC, forcing or compressing the air/fuel mixture into the much smaller combustion chamber of the cylinder head. The movement of the piston also causes turbulence in the cylinder, which mixes the air/fuel mixture further, allowing more of the chemical energy in the fuel to be released during the power stroke.
While it is the fuel that stores the chemical energy that drives the engine, it is the air that allows the fuel to burn and release its energy. Too little air leads to a rich fuel mixture that does not burn completely and does not release all the energy in the fuel, robbing the engine of power and economy. Too much air leads to a lean fuel mixture that burns too quickly. When the air/fuel mixture burns too quickly, it spends its energy too soon and creates too much pressure too quickly. This can cause irreparable damage to the engine. The chemically ideal ratio of air to fuel is 14,7 parts air to 1 part fuel (14,7:1) and is referred to as the stoichiometric condition. However, the air/fuel mixture requirements of the internal combustion engine are influenced by RPM, engine load and temperature. Heat is required for fuel vaporization. Therefore, in cold start conditions, a richer mixture is required and at full throttle, or wide-open-throttle (WOT), a leaner mixture is required. This is why fuel injection has a major advantage over the carburetor; it can provide the correct air/fuel mixture under varying conditions.
The Power Stroke
The power stroke is the next stroke in the Otto cycle and is also the start of the second revolution of the engine. The intake and compression strokes take one complete revolution of the engine, while the power stroke and the exhaust stroke take another complete revolution; in other words, the four-stroke cycle is completed over two revolutions of the engine.
Just before the start of the power stroke, the spark plug fires, igniting the compressed air/fuel mixture in the combustion chamber which then burns in a fairly controlled manner. This causes an increase in temperature and an expansion of the gasses in the combustion chamber, and ultimately increases the pressure in the combustion chamber. This pressure increases progressively and acts upon the top of the piston, pushing it down the cylinder to BDC. It is important to note that the pressure increases progressively until peak cylinder pressure is reached slightly past TDC, at approximately 12° to 14° after TDC. If peak pressure is reached at TDC, the pressure would be applied directly on crankshaft and its bearing, which would absorb a large amount of the power being produced.
The pressure pushing down on the piston and forces the crankshaft to rotate, converting the chemical energy in the fuel to mechanical energy. Unfortunately, the internal combustion engine is not very efficient and a lot of this energy is lost through heat that is absorbed by the engine, and lost through the exhaust system. Though the heat energy that is lost through the exhaust can be used to drive a turbocharger so it can't be all that bad, can it?
The Exhaust Stroke
The burnt air/fuel mixture is expelled from the engine during the exhaust stroke. The exhaust valve opens slightly before the stroke begins. With the exhaust valve open, the movement of the piston from BDC to TDC forces the burnt air/fuel mixture through the exhaust valve and out of the engine. Usually, the exhaust valve opens slightly before the stroke begins and closes slightly after the stroke ends, allowing the engine to expel as much burnt air/fuel mixture as possible. Any burnt air/fuel mixture or exhaust gasses that remain in the combustion chamber after the exhaust stroke will contaminate the fresh air/fuel mixture that in drawn into the cylinder on the next intake stroke, and will effectively reduce engine power.
The intake and exhaust valves can open slightly before the start of their respective strokes and can close slightly after the end of their respective strokes because the linear movement of the piston slows down dramatically to a dead stop as it reaches TDC and BDC. However, the opening and closing of the valves must occur at exactly the correct moment to ensure maximum engine power, particularly as the fresh air/fuel mixture coming into the cylinder just before the end of the exhaust stroke helps push out more of the burnt air/fuel from the previous cycle through a process called "scavenging". We discuss valve timing in our section on camshafts and cylinder heads; but in our next section on engine basics, we'll show you how to increase engine power.