Fuels — typical calorific value of automotive diesel Select the closest standard lower heating value for diesel fuel used in compression-ignition engines.
Correct Answer: 42.5 MJ/kg
Introduction / Context:The calorific value of diesel determines available energy per kilogram and influences specific fuel consumption and engine mapping. Diesel typically contains heavier hydrocarbons than petrol, giving a similar or slightly higher LHV depending on formulation.
Given Data / Assumptions:
- Values refer to LHV in MJ/kg for pump diesel.
- No biodiesel blending or seasonal extreme variations assumed.
- Standard ambient conditions.
Concept / Approach:Typical LHV for diesel fuel is near 42–43 MJ/kg. This reflects the average hydrogen-to-carbon ratio and distillation range. While HHV is higher, engine calculations for CI engines usually rely on LHV due to exhaust water remaining as vapor.
Step-by-Step Solution:
Recall the widely used design value for diesel ≈ 42–43 MJ/kg (LHV).Identify the option closest to this canonical figure.Select 42.5 MJ/kg as the representative value.Verification / Alternative check:Thermochemical tables and engine texts list diesel LHV commonly around 42.5 MJ/kg; local fuel specs vary slightly.
Why Other Options Are Wrong:
- 36.5 or 38.5 MJ/kg: lower than typical for diesel; closer to some oxygenated fuels.
- 45.5 MJ/kg: on the high side for LHV; more akin to HHV claims or specific kerosene fractions.
- 40.0 MJ/kg: possible for certain blends, but not the canonical design figure.
Common Pitfalls:Confusing HHV and LHV; ignoring the effect of biodiesel (which slightly lowers LHV).
Final Answer:
42.5 MJ/kg