What the Compression Stroke Does in an Engine

The compression stroke squeezes the air-fuel charge (gasoline engines) or just air (diesel engines) into a smaller volume, sharply raising pressure and temperature so the mixture will ignite efficiently and produce maximum power on the next stroke. It is the second step in a four-stroke engine’s cycle and is essential to fuel efficiency, torque, emissions control, and reliable combustion.

How the Compression Stroke Works

During the compression stroke, the piston travels upward from bottom dead center (BDC) to top dead center (TDC) with both intake and exhaust valves closed. This upward movement compresses the trapped charge. In a gasoline engine, a timed spark occurs near the end of the stroke to ignite the mixture; in a diesel engine, fuel is injected into the hot, highly compressed air, where it self-ignites. The rapid rise in pressure and temperature during this stroke sets the stage for the power stroke that follows.

Gasoline vs. Diesel: Two Paths to Ignition

Gasoline engines rely on spark ignition and typically use moderate static compression ratios (roughly 9:1 to 13:1 in modern designs) to balance efficiency with knock resistance. Direct injection and precise control of valve timing allow higher effective compression while avoiding detonation. Diesel engines use compression ignition, so they compress only air to very high pressures and temperatures; typical compression ratios run from about 14:1 to over 20:1. The hotter, denser air ignites fuel as it’s injected at the end of the stroke, enabling high thermal efficiency and strong low-end torque.

Why Compression Matters

Compression is central to engine efficiency and performance. As compression ratio increases, thermal efficiency improves (as described by the idealized Otto and Diesel cycles), meaning more of the fuel’s energy is converted into useful work. However, gasoline engines are limited by knock (uncontrolled auto-ignition), which is managed through fuel octane, combustion-chamber design, direct injection, cooled exhaust gas recirculation (EGR), and sophisticated ignition and boost control. Some modern designs, including variable compression ratio systems, adjust effective compression to optimize power, economy, and emissions in real time.

The key outcomes of the compression stroke can be summarized to highlight why it is indispensable to modern engines.

• Pressure rise: Squeezing the charge raises cylinder pressure, enabling a powerful subsequent expansion (power) stroke.
• Temperature rise: Heating from compression improves mixture readiness for ignition; in diesels, it’s critical for auto-ignition.
• Mixture preparation: Swirl/tumble and squish zones enhance air-fuel mixing and flame speed in gasoline engines.
• Sealing check: Any leakage past valves, head gasket, or piston rings reduces compression and performance.
• Efficiency gain: Higher effective compression generally improves fuel economy and torque within knock and durability limits.

Together, these effects ensure the engine produces consistent power while meeting efficiency and emissions targets.

Where It Fits in the Four-Stroke Cycle

Understanding the compression stroke is easier in the context of the complete four-stroke sequence used by most modern automotive engines.

1. Intake: The intake valve opens, the piston moves down, and the cylinder fills with fresh air (and fuel in port-injected or carbureted engines).
2. Compression: Both valves close, the piston moves up, and the charge is compressed for ignition readiness.
3. Power (Combustion/Expansion): The spark ignites the mixture (gasoline) or injected fuel ignites in hot compressed air (diesel), forcing the piston down.
4. Exhaust: The exhaust valve opens, the piston moves up, and spent gases are expelled.

Each stroke prepares the next; compression is the critical bridge between filling the cylinder and extracting useful work from combustion.

Signs of Compression Problems

When compression is weak or uneven between cylinders, engines lose power, idle roughly, and may misfire. The following issues commonly cause compression-related faults.

• Worn or stuck piston rings allowing blow-by into the crankcase.
• Leaking or burned valves that fail to seal the combustion chamber.
• Blown head gasket causing pressure loss between cylinders or to coolant.
• Incorrect valve timing (e.g., slipped timing belt/chain or cam phaser fault) leaving valves open when they should be closed.
• Cracked head/block or damaged piston reducing sealing capability.

Addressing compression issues typically starts with testing to localize the leak path before mechanical repair.

Measuring Compression and Typical Values

Two common diagnostic tools are the compression test and the leak-down test. A compression test measures peak cranking pressure per cylinder; a leak-down test measures the percentage of air escaping from a cylinder at a set pressure, pinpointing leakage past rings, valves, or gaskets.

Before listing typical values, it helps to distinguish static compression ratio (a geometric ratio) from measured cranking compression (pressure while the starter cranks). The latter varies with engine speed, temperature, altitude, cam timing, and throttle position during the test.

• Gasoline engines: Often 120–200+ psi (8–14 bar) at cranking, with even readings across cylinders more important than the absolute number.
• Diesel engines: Commonly 350–600 psi (24–41 bar) or higher, reflecting their much higher compression ratios.
• Altitude effect: Higher elevations reduce measured compression due to lower ambient pressure.
• Cam/valve timing: Aggressive valve timing or variable valve timing positions can lower cranking readings at low speed.

While ranges vary by design, consistency between cylinders and alignment with manufacturer specifications are the key diagnostics.

What Influences the Effectiveness of the Compression Stroke

Several design and maintenance factors determine how well a cylinder compresses its charge and prepares for clean, powerful combustion.

• Sealing quality: Piston rings, cylinder wall finish, valve seats, and head gasket integrity.
• Valve timing: Correct cam timing and functional VVT ensure valves are closed during compression.
• Intake conditions: Charge temperature, humidity, and boost pressure (turbo/supercharged engines) change density and knock tendency.
• Combustion chamber design: Swirl/tumble and squish areas enhance mixture motion and flame propagation.
• Control strategies: Direct injection timing, EGR rates, and ignition timing tune effective compression and knock resistance.
• Carbon deposits: Can raise effective compression and hot spots, increasing knock risk in gasoline engines.
• Variable compression ratio systems: Newer mechanisms can mechanically alter compression to balance power and efficiency.

Managing these variables allows manufacturers to extract more power and efficiency while meeting emissions standards and ensuring durability.

Summary

The compression stroke compresses the cylinder’s charge, elevating pressure and temperature so combustion can occur efficiently and forcefully on the power stroke. In gasoline engines it prepares the air-fuel mix for a spark; in diesels it heats air for fuel auto-ignition. Its effectiveness—shaped by sealing, valve timing, chamber design, control strategies, and, increasingly, variable compression technologies—directly determines engine performance, efficiency, and emissions.

What happens at the end of the compression stroke?

Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle.

What is true regarding the compression stroke?

During the compression stroke, the piston moves upward, compressing the air-fuel mixture in the cylinder after the intake stroke is complete. Both the intake and exhaust valves are closed during this stroke, which increases the pressure and temperature of the mixture. This compressed mixture is then ready to be ignited by the spark plug, leading to the power stroke. 
Here’s a breakdown of the compression stroke:

• Piston Movement: The piston travels from Bottom Dead Center (BDC) to Top Dead Center (TDC), moving up in the cylinder. 
• Valve Position: Both the intake and exhaust valves are closed to trap the air-fuel mixture. 
• Purpose: The primary goal of the compression stroke is to increase the pressure and temperature of the air-fuel mixture by reducing its volume. 
• Outcome: A more compressed mixture is more volatile and will burn more explosively when ignited, generating more power. 

In essence, the compression stroke prepares the mixture for efficient combustion by squeezing it into a small volume, which raises its temperature and pressure.

What happens during the combustion stroke?

The increased speed of the piston during the combustion stroke generates torque in the engine’s crankshaft. This torque powers the transmission, which in turn powers your wheels. The cylinder’s intake valve and exhaust valve both remain closed during the combustion stroke.

What happens in a compression stroke?

During the compression stroke of a four-stroke engine, the piston moves upward, compressing the trapped air-fuel mixture within the cylinder as both the intake and exhaust valves remain closed. This upward movement, after the intake stroke has filled the cylinder with the mixture, increases the pressure and temperature of the mixture. The compression stroke ends at the Top Dead Center (TDC), a point where the mixture is highly compressed and ready to be ignited by the spark plug.
 
Key events of the compression stroke:

1. Valves are closed: Opens in new tabJust after the intake stroke finishes and the piston is at its lowest point (Bottom Dead Center), the intake valve closes, and the exhaust valve remains closed. 
2. Piston moves up: Opens in new tabThe upward movement of the piston begins to reduce the volume inside the cylinder. 
3. Mixture is compressed: Opens in new tabBecause the valves are closed, the air-fuel mixture has nowhere to escape and is squeezed into a much smaller space. 
4. Pressure and temperature increase: Opens in new tabThis compression significantly raises the pressure and temperature of the air-fuel mixture. 
5. Preparation for ignition: Opens in new tabThe cylinder is now sealed and contains a highly compressed, volatile fuel-air mixture that is set to ignite in the subsequent power stroke. 
6. Reaches Top Dead Center (TDC): Opens in new tabThe compression stroke concludes when the piston reaches its highest point in the cylinder, known as Top Dead Center.