Relative cylinder size for gas engines versus petrol engines Considering typical design practice and energy density of the inducted charge, the cylinder of a gas-fuelled engine (running on gaseous fuel) is generally made ________ than that of a comparable petrol engine to achieve similar power output.

Introduction / Context:
Engine sizing is influenced by the energy per unit charge and volumetric efficiency. Gas engines (using gaseous fuels such as natural gas or producer gas) often deliver less energy per inducted cylinder charge than petrol engines under similar conditions.

Given Data / Assumptions:

• Comparing engines of similar technology and speed class.
• Gas engine uses a gaseous fuel–air mixture; petrol engine uses gasoline–air mixture.
• Goal: achieve similar brake power.

Concept / Approach:
Gaseous fuels displace part of the intake air volume, lowering the mass of oxygen available per cycle relative to liquid-fuelled SI engines where only vaporised fuel occupies minimal volume. Additionally, some gaseous fuels have lower volumetric energy density. To maintain comparable power, designers increase cylinder displacement (larger cylinder) or boost intake density.

Step-by-Step Solution:
Recognise reduced oxygen/fuel energy per charge for gaseous mixtures.Compensate by increasing displacement (larger cylinder dimensions).Therefore select “larger.”

Verification / Alternative check:
Practical engine catalogues show natural gas variants often having lower specific power or larger displacement than gasoline counterparts at the same ratings.

Why Other Options Are Wrong:

• Smaller or same size: would reduce power further unless compensated by boosting.
• “Variable” options do not address the general design trend taught in fundamentals.

Common Pitfalls:
Confusing gaseous-fuel turbocharged engines (which can recover power) with naturally aspirated designs; the general statement refers to unboosted or lightly boosted cases.

Final Answer:
larger