Steam engine fundamentals: In a classic reciprocating steam engine, fuel is burnt in a separate furnace/boiler and steam is then admitted into the cylinder for expansion. Combustion of fuel takes place __________ the engine cylinder of a steam engine.

Introduction / Context:
A steam engine is a prime example of an external combustion system. Understanding where combustion occurs helps distinguish steam engines from internal combustion engines such as diesel and petrol engines, and is foundational for boiler–engine system design and safety.

Given Data / Assumptions:

• The working fluid in the engine cylinder is steam generated in a separate boiler.
• Fuel (coal, oil, gas, biomass) is burned in a furnace that is part of the boiler, not the cylinder.
• The engine cylinder only receives steam for doing mechanical work on the piston.

Concept / Approach:
External combustion means the chemical energy release from fuel happens outside the power cylinder. Heat from combustion converts water to steam in the boiler. The steam then expands in the engine cylinder, creating pressure forces on the piston without any flame or combustion taking place inside the cylinder itself.

Step-by-Step Solution:
Identify the energy conversion locations: boiler (chemical to thermal) and engine (thermal to mechanical).Note the cylinder environment: contains steam only, plus exhaust to condenser or atmosphere.Recognize safety and design implications: no spark, injector, or mixing of air–fuel inside the cylinder.Therefore, combustion occurs outside the cylinder, in the boiler furnace.

Verification / Alternative check:
Compare with diesel/petrol engines where fuel–air burns inside the cylinder; the steam engine lacks an internal ignition event.

Why Other Options Are Wrong:
Inside/partly inside: contradicts external combustion definition.

Inside only during starting/high pressure: there is never in-cylinder combustion in a true steam engine.

Common Pitfalls:
Confusing steam engines with internal combustion engines due to similar piston–crank mechanisms.

Final Answer:
outside