Thermal efficiency benchmark for spark-ignition engines In standard automotive practice, the brake thermal efficiency of typical petrol (gasoline) and gas (spark-ignition) engines is approximately what value under steady, well-tuned operating conditions?

Introduction / Context:
Brake thermal efficiency is a key performance metric for internal combustion engines. It represents the fraction of the chemical energy in the fuel that is converted into useful shaft work at the crankshaft. For spark-ignition engines (petrol and gas), understanding the typical efficiency band helps students interpret dyno data, compare technologies, and set realistic expectations for improvements such as turbocharging or hybridization.

Given Data / Assumptions:

• Spark-ignition engines operating at steady load and speed.
• Conventional fuels: petrol (gasoline) or gaseous fuels (e.g., CNG).
• Well-maintained engine with modern but not exotic technology; no waste-heat recovery.

Concept / Approach:
Brake thermal efficiency = brake power output divided by the rate of fuel energy input. SI engines typically exhibit lower efficiency than CI engines due to throttling losses, lower compression ratios, and flame-speed/knock constraints. Historically, naturally aspirated SI engines achieve about 20–30% at best efficiency point; modern improvements can touch the low-to-mid 30% range, but a representative single figure for learning and quick estimates is around 30%.

Step-by-Step Solution:
Define BTE = P_brake / (m_dot_fuel * LHV).Identify known range for SI: roughly 20%–35% depending on design and operating point.Choose a reasonable representative value used in textbooks and exam problems: about 30%.Therefore, the closest option is 30%.

Verification / Alternative check:
Manufacturer and research maps often show best efficiency islands for SI engines near 30% BTE, with lower values at low load due to throttling. This corroborates the educational benchmark.

Why Other Options Are Wrong:
15% is typical of very light load or old/poorly tuned engines, not a benchmark. 50% and 70% exceed even advanced SI engine capabilities; such figures are associated with large combined-cycle plants or fuel cells, not standard SI engines.

Common Pitfalls:
Confusing indicated efficiency with brake efficiency, or quoting peak lab results as typical road values. Real-world averages are lower than peak BTE at the best efficiency point.

Final Answer:
30%