For which of the following reasons, do the indirect injection diesel engines have higher specific output compared to direct injection diesel engines? I. They have lower surface to volume ratio. II. They run at higher speeds. III. They have higher air utilization factor. IV. They have lower relative heat loss. Select the correct answer using the code given below:
(a) I and II
(b) II only
(c) II and III
(d) III and IV
The Correct Answer and Explanation is :
The correct answer is (a) I and II.
Explanation:
Indirect injection (IDI) diesel engines have higher specific output compared to direct injection (DI) diesel engines for a variety of reasons, most of which are tied to the design and operating characteristics of the engine. Let’s break down the four given statements:
- Lower Surface to Volume Ratio (I): Indirect injection engines generally have a pre-chamber where the air-fuel mixture is initially prepared before being injected into the combustion chamber. This setup results in a lower surface-to-volume ratio compared to DI engines, where the air-fuel mixture is injected directly into the combustion chamber. A lower surface-to-volume ratio reduces heat loss because there is less surface area through which heat can escape, improving combustion efficiency. This also leads to higher specific output in IDI engines since the engine operates more efficiently, with less energy lost to heat.
- Running at Higher Speeds (II): Indirect injection engines are often designed to run at higher engine speeds compared to direct injection engines. This is because the pre-chamber allows for smoother combustion and quicker combustion cycles, which facilitates faster engine operation. Higher engine speeds generally result in higher specific output, as more power is produced per unit of engine displacement. In IDI engines, running at higher speeds allows for higher revolutions per minute (RPM) without compromising engine stability or reliability, which enhances specific output.
- Higher Air Utilization Factor (III): The air utilization factor refers to the efficiency with which the engine uses the available air for combustion. While this is crucial for overall engine performance, it does not directly contribute to the higher specific output of IDI engines. IDI engines generally have a slightly lower air utilization factor compared to DI engines because of the nature of their combustion chambers. Direct injection engines typically have more efficient air-fuel mixing and combustion.
- Lower Relative Heat Loss (IV): Although IDI engines do tend to have lower relative heat losses compared to some other engine designs, this is not the primary factor that contributes to their higher specific output. The primary contributors are the reduced surface-to-volume ratio and the ability to operate at higher speeds.
Thus, the key factors are I (lower surface-to-volume ratio) and II (higher engine speeds), leading to the conclusion that the correct answer is (a) I and II.