A 150-Year Heritage

First, a point of clarification: if you’ve been accustomed to referring to a 4-cycle engine, that’s a misnomer. It’s properly termed a 4-stroke-cycle engine, or 4-stroke, for short. Each piston travels through four strokes to complete a single cycle. Any given cycle is the succession of operations that are constantly being repeated in a cylinder when the engine is running. And a stroke can be defined as the piston moving from the point where it’s the highest in the cylinder to the lowest point in the cylinder, or vice-versa.

You may have heard some engine experts describe this as the “Otto” cycle. This is a reference to Nicholas A. Otto, who patented the technology in 1876. Its first practical demonstration happened two years later in 1878. However, if you want to get really technical and give credit to the original inventor of the 4-stroke-cycle process, that would be Alphonse Beau De Rochas, a French railroad engineer. He published a pamphlet describing the cycle in 1862 — more than 150 years ago and a decade and a half before Otto obtained the patent.

Now here’s something that’s very important to understand — the following descriptions are theoretical, not real world. This tech tip describes how the four strokes would occur under ideal lab conditions. It even has a name: Air-Standard Cycle Analysis. This name indicates that the following cycles would take place in a working medium of plain air — with no chemical reaction, no heat gained or lost (adiabatic), and no mass to the medium and therefore no inertia. None of this is truly possible, but it makes it easier to understand the 4-stroke-cycle process.

A Stroke of Genius

Let’s take a closer look at the four strokes: intake, compression, power, and exhaust.

Intake stroke — The first stroke is called the “intake” stroke. It begins when the piston is at top dead center (TDC). At this time, the intake valve(s) are open and the piston has just begun moving downward. The pressure differential caused by the downward moving piston and the 200 miles of atmosphere causing 14.7 pounds per square inch (psi) ambient pressure — at sea level — forces the air/fuel mixture through the lower pressure intake tract and into the combustion chamber and cylinder.

Compression stroke — The second stroke is known as the “compression” stroke. Once the piston has reached bottom dead center (BDC) — or, in other words, the lowest the piston can go — it begins moving upward. At this point, both valves are closed. The rising piston reduces the total volume available in the cylinder/combustion chamber. This leads to a rise in pressure that’s proportional to the amount of air that’s compressed.

Power (Combustion) stroke — The third stroke is labeled the “power” stroke. Once the piston has again reached TDC, the power stroke begins. Both valves are closed and the air/fuel mixture is ignited. This forces the piston down.

Exhaust stroke — The fourth stroke is referred to as the “exhaust” stroke. It begins when the piston has reached BDC. The exhaust valve is open during this stroke. The piston begins moving up and forces the exhaust gases out of the cylinder. When the piston reaches TDC, the cycle starts over again at the intake stroke.

Remember, the four strokes are: intake, compression, power (combustion), and exhaust.

Real World: Why a 6-Stroke Cycle Is More Accurate

In reality, if your engine’s valves opened at exactly TDC or BDC, the engine would run poorly and wouldn’t be able to achieve a high degree of revs. Because heat is created in the real world and the working medium is not just air, but rather an air/fuel mixture, a working mechanism is required to accommodate this. As a result, the actual strokes don’t start at TDC or BDC. They’re defined by the points at which the valves open and close.

Intake Stroke

This stroke begins when the intake valve opens. This point in the cylinder may be described by a number of degrees before top dead center (BTDC). For example, let’s assume that the intake stroke starts 30 degrees BTDC. When the intake valve(s) start(s) opening, the exhaust valve(s) will also open, but will close quickly. The intake stroke will continue as the piston reaches TDC and on until it is at BDC, finally ending when the intake valve closes a number of degrees after bottom dead center (ABDC). The reason the valve closes ABDC is because the inertia of the incoming air/fuel charge, makes it possible to use this inertia to fill the cylinder to more than 100% (by weight) — a sort of inertial supercharging.

Compression Stroke

The compression stroke starts as soon as the intake valve closes — at, let’s say, 60 degrees ABDC. The piston is now rising in the cylinder. When it reaches TDC, the compression stroke stops. The reason is that once the piston has crested and begins moving downward, the medium would no longer be at maximum compression because the piston is allowing the volume of the combustion chamber/cylinder to expand. As you might imagine, the engine can’t compress the charge until both valves are closed, meaning there’s nowhere for the gases to go. The compression ratio that’s measured from the point where the intake valve closes is called the effective compression ratio.

Power Stroke

The power stroke begins at TDC. Sometime before TDC, the spark plug will ignite the air/fuel mixture in the cylinder. For this discussion, assume that this has occurred 20 degrees BTDC. The cylinder pressure and temperature is now rising rapidly due to the burning of the air/fuel mixture and, once the piston is moving downward, these gases start having an impact on the piston. This drives it powerfully downward and the connection rod and crankshaft convert the energy from reciprocating to rotary motion. The power stroke ends when the exhaust valve opens.

Exhaust Stroke

The exhaust stroke begins the moment the exhaust valve opens. In this illustration, let’s say it happens 60 degrees before bottom dead center (BBDC). The stroke continues as the piston reaches BDC, while it moves from BDC back up to TDC, and then on until the exhaust valve closes. Assume that it closes 30 degrees after top dead center (ATDC).

The “5th” Stroke

If you’ve been paying close attention to this description, you may have noticed there’s a period of time when both the intake and exhaust valves are open. This is sometimes referred to as the “5th” stroke. It’s also known as “overlap” or “valve overlap.” Overlap occurs from the time the intake valve opens until the exhaust valve closes. As it begins, the exhaust valve is open a little and the intake valve is just starting to open. As the intake opens wider, the exhaust valve narrows. Overlap ends when the exhaust valve closes completely. During overlap, the piston is at TDC and is not moving very much relative to the number of degrees the crankshaft is turning. This phenomenon when the piston’s motion has nearly stopped is known as “piston dwell” or just “dwell.” Having both the intake and exhaust valves open, combined with dwell, creates a time when the air/fuel mixture is entering through the intake valve and the exhaust gases are moving out the exhaust valve. The inertia of the air/fuel charge helps it push the burnt gases that remain in the cylinder, known as “clearance” gases, out the exhaust. This is important because any burnt gases that are allowed to stay in the cylinder take up space where the incoming air/fuel mixture could be. In addition, the extremely high temperatures of the clearance gases would heat the incoming air/fuel charge, making it less dense. It could also lead to detonation. Using the inertia of the gases leaving the cylinder, it’s possible to actually help the incoming air/fuel mixture to be quickly and efficiently drawn into the cylinder. It’s also possible to use pressure waves that are traveling up and down the exhaust to help draw in the fresh air/fuel mixture charge.

The “6th” Stroke

The time available for generating power is from TDC to the point that the exhaust valve opens — 60 degrees BBDC in our example. This only gives 120 degrees in which the pressure in the cylinder pushes the piston down and creates power. This may seem counterproductive, but in reality it allows the engine to make more power. As the exhaust valve opens, any pressure left in the cylinder is blown out the exhaust. This period is known as the “blowdown period” or simply “blowdown.” The idea is to force as much pressure as possible out through the exhaust before the piston hits BDC and begins moving upward. If the exhaust valve were to open at BDC, the piston would be responsible for pushing all the burnt gases out the exhaust valve. The pressure in the cylinder would actually be pushing against the piston as it comes up, causing it to resist moving upward. This resistance is termed “pumping losses.” A pumping loss occurs anytime the engine has pressure above ambient environmental pressure acting on the piston while it is moving upward. Therefore, it’s extremely important to push as much pressure out of the cylinder before the piston begins moving up on the exhaust stroke. It’s a fine balance between holding the pressure in the cylinder to force the piston down, creating power, and then opening the exhaust valve to get the pressure out of the cylinder to avoid pumping losses. This compromise is one factor to consider when choosing a camshaft.

Contact the Experts

While you may not use your knowledge of the 4-stroke-cycle engine day-to-day, the more you know about how an engine works, the easier it will be to address certain issues yourself or have more informed conversations about your car with your favorite technician.

To make an appointment to discuss this or any other service or performance upgrade requirements, contact Jason McCallum or your preferred Borelli Motor Sports technician at (408) 770-1220.