Operating cost comparison – petrol vs. diesel For similar output and duty, how does the running (operating) cost of a petrol engine compare with that of a diesel engine?

Introduction / Context:
Running cost encompasses the fuel consumed per unit of useful work and the price of that fuel. Diesel engines generally achieve higher thermal efficiency and lower brake specific fuel consumption than spark-ignition petrol engines for comparable duty cycles.

Given Data / Assumptions:

• Comparable displacement and load profiles.
• Conventional fuels: petrol (gasoline) vs. diesel fuel.
• Maintenance costs excluded; focus on fuel cost per work delivered.

Concept / Approach:
Diesel engines operate at higher compression ratios and avoid throttling losses, giving better efficiency, especially at part load. Consequently, for a given task, diesel engines consume less fuel energy. Even when fuel prices vary, the combination of lower specific consumption and often lower energy cost per unit can yield lower running cost for diesel. Thus, petrol engines typically have higher running costs for equivalent output.

Step-by-Step Solution:
Compare BSFC: diesel < petrol in g/kWh.Consider fuel price and energy content: diesel often competitive per kWh delivered.Result: cost per unit work for petrol > diesel → petrol has higher running cost.

Verification / Alternative check:
Fleet and generator operating data commonly show better fuel economy for diesel under steady loads; automotive comparisons reflect similar trends on highway cycles.

Why Other Options Are Wrong:

• “Lower” contradicts typical BSFC differences.
• “Identical” ignores clear thermodynamic and mechanical distinctions.
• “Cannot be compared” is overly cautious; general engineering practice supports the comparison.
• Reversing the trend (diesel higher cost) does not reflect standard outcomes for similar duty.

Common Pitfalls:
Confusing capital/maintenance costs with running cost; while those can differ, the question specifically targets fuel-related running cost.

Final Answer:
higher