What is drawn in the cylinder of diesel engine during suction stroke?
Pure air alone
Understanding the Diesel Engine Suction Stroke
Internal combustion engines, including diesel engines, typically operate through a cycle of four strokes. These strokes are: suction (or intake), compression, power (or expansion), and exhaust. Each stroke plays a crucial role in converting fuel energy into mechanical work. Let's focus on the initial stroke in a diesel engine – the suction stroke.
What Enters the Cylinder During Suction?
During the suction stroke of a diesel engine, the piston moves downwards inside the cylinder. This downward movement increases the volume within the cylinder, creating a pressure lower than the atmospheric pressure. The intake valve opens, and the higher atmospheric pressure forces a working fluid into the cylinder.
A key difference between a diesel engine and a petrol (or spark-ignition) engine lies in what this working fluid is during the suction stroke.
- In a petrol engine, a pre-mixed mixture of air and fuel is drawn into the cylinder during suction.
- In a diesel engine, however, only pure air alone is drawn into the cylinder during the suction stroke.
The diesel fuel is injected directly into the combustion chamber later, during the compression stroke, not mixed with the air before it enters the cylinder.
Why Only Pure Air in a Diesel Engine Suction Stroke?
The reason only pure air alone is drawn into the cylinder of a diesel engine during the suction stroke is related to its combustion process. Diesel engines rely on compression ignition. During the compression stroke, the piston moves upwards, compressing the air drawn in during suction to a very high pressure and temperature. Once the air reaches a sufficiently high temperature, diesel fuel is injected into this hot, compressed air. The fuel ignites spontaneously upon contact with the hot air without needing a spark plug (as in petrol engines).
If fuel were present during the entire compression stroke from the beginning (like in a petrol engine), it could potentially ignite prematurely under the very high compression ratios used in diesel engines, leading to engine knocking or damage. By drawing in only pure air alone during suction and injecting fuel later, the diesel engine controls the ignition process precisely.
Comparing Suction Stroke: Diesel vs. Petrol Engines
Here is a simple comparison of what is drawn into the cylinder during the suction stroke for the two common types of internal combustion engines:
| Engine Type | Substance Drawn In During Suction Stroke |
|---|---|
| Diesel Engine | Pure air alone |
| Petrol Engine | Mixture of air and fuel |
The Diesel Engine Suction Stroke Process
Let's break down the steps during the suction stroke in a diesel engine:
- The piston is at the top of its travel (Top Dead Center or TDC).
- The intake valve opens.
- The piston moves downwards towards the bottom of its travel (Bottom Dead Center or BDC).
- The volume inside the cylinder increases, creating vacuum pressure.
- Atmospheric pressure pushes pure air alone into the cylinder through the open intake valve.
- The piston reaches BDC, and the cylinder is filled with pure air.
- The intake valve closes, sealing the cylinder for the next stroke (compression).
Therefore, the substance drawn into the cylinder of a diesel engine during the suction stroke is indeed pure air alone.
Revision Table: Diesel Engine Strokes
| Stroke | Piston Movement | Valves (Intake/Exhaust) | Process | Cylinder Content (Start of Stroke) | Cylinder Content (End of Stroke) |
|---|---|---|---|---|---|
| Suction (Intake) | TDC to BDC (Down) | Intake Open, Exhaust Closed | Draws in working fluid | Residual Exhaust Gas | Pure Air Alone |
| Compression | BDC to TDC (Up) | Intake Closed, Exhaust Closed | Compresses working fluid | Pure Air | Compressed, Hot Air |
| Power (Expansion) | TDC to BDC (Down) | Intake Closed, Exhaust Closed | Fuel injected and ignites, expanding gases push piston | Compressed Air + Injected Fuel | Hot Exhaust Gases |
| Exhaust | BDC to TDC (Up) | Intake Closed, Exhaust Open | Expels exhaust gases | Hot Exhaust Gases | Residual Exhaust Gas |
Additional Information on Diesel Engine Operation
Understanding the suction stroke of a diesel engine is fundamental to grasping its overall operation. Here are some related points:
- Fuel Injection: As mentioned, diesel fuel is injected later in the cycle (during compression or slightly before power stroke). This is done using a high-pressure fuel injector.
- Compression Ratio: Diesel engines typically have much higher compression ratios than petrol engines (e.g., 14:1 to 25:1 compared to 8:1 to 12:1 for petrol). This high compression is necessary to raise the air temperature sufficiently for auto-ignition of the diesel fuel.
- Air-Fuel Ratio Control: Unlike petrol engines which often run close to stoichiometric ratios for efficient combustion and emissions control using a catalytic converter, diesel engines often run lean (excess air). The power output in a diesel engine is primarily controlled by the amount of fuel injected, not by throttling the air intake (like in many petrol engines).
- Turbocharging: Diesel engines often use turbochargers to force more air into the cylinder during the suction stroke (this is called forced induction), increasing power output and efficiency.
The correct answer is that pure air alone is drawn into the cylinder of a diesel engine during the suction stroke.
Important Questions from Introduction To Engine
In a multi-cylinder engine, which of the following arrangements is considered to be the simplest?
In diesel engines, fuel and air are mixed in the __________.
There are ________ types of Gas Turbines.
- What is the function of an engine?
- If the air and fuel mixture burns inside the engine cylinder, that engine is called as:
