Comparing Exhaust Temperatures – Petrol (SI) vs. Diesel (CI) For the same maximum pressure and the same heat input, how does the exhaust gas temperature of a petrol (spark-ignition) engine compare with that of a diesel (compression-ignition) engine?

Introduction / Context:
Exhaust temperature influences turbine matching, catalyst durability, and emission control strategies. Knowing which prime mover tends to have hotter exhaust for comparable limits helps in thermal management and material selection.

Given Data / Assumptions:

• Same maximum cylinder pressure and identical heat input conditions are assumed.
• Typical operating strategies: petrol engines usually run near stoichiometric; diesels operate lean with higher expansion ratios.
• Friction and heat losses are of comparable order for the comparison.

Concept / Approach:

Diesel engines generally have higher compression and effective expansion ratios, extracting more work from the hot gases before exhaust valve opening. They also operate with excess air (lean mixtures), which lowers combustion temperatures and increases specific heat capacity of the working charge. Conversely, petrol engines often run near stoichiometric and with lower expansion ratios, leaving more sensible energy in the exhaust. Therefore, for the same maximum pressure and heat input, petrol engines tend to have higher exhaust temperatures than diesels.

Step-by-Step Solution:

Verification / Alternative check:

Engine test data routinely show lower exhaust gas temperatures for naturally aspirated diesels than for comparable SI engines at similar load fractions, supporting the conceptual reasoning.

Why Other Options Are Wrong:

'Less' contradicts typical thermodynamic outcomes; 'equal' is unlikely across wide operating ranges; altitude affects both similarly but does not reverse the trend under stated constraints.

Common Pitfalls:

Assuming turbocharging alone dictates EGT; while boosting changes details, the SI vs. CI trend under the given premise remains.

Final Answer:

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