Engine Design and Model Manufacture

Engine design and model manufacture – Formula, calculations, good practice and general know-how. This is a bit of a mix of steam engine and internal combustion engine design. Information and pages will get gradually added, but if there is something you would like to know or contribute please do drop us a line. Lots of the pages are based on my knowledge gained from making model engines for the last 40 years…

Air Intake System – this ducts clean air to the engine via the intake vakes.

bore and stroke ratio

Bore & Stroke – The engine bore and stroke determines the swept volume of the engine. The ratio of the bore to stroke determines some of the fundamental characteristics such as maximum torque and ability to rev. Plotting engine stroke versus engine bore is a visual way to explore some of the resultant engine designs.

Bottom Dead Centre – The position of the crank when the piston is in its closest position to the crankshaft, in its farthest position from the cylinder head. Abbreviated bdc.

piston, conrod, crank, cylinder parts

The capacity of an engine is a simple calculation using the bore, stroke and number of cylinders.

carnot cycle

Carnot Cycle – In 1824 Carnot showed that the amount of heat which could be converted into mechanical work by an ideal perfect heat engine using a perfect gas as the working fluid, depended solely upon the working range of temperature.

connecting rod

There are many different conrod (connecting rod) designs amongst all of the different types of engine type and formats. The mass, length and strength of the conrod are a number of important factors in the overall design.

conrod end bearing designs
crank slider mechanism

In steam engines it is normal to have a two part conrod and then to have a slider mechanism to support the joint in the conrod. This results in a number of different sliding joint designs.

cooling fins

There are some simple rules for the spacing of Cooling Fins for the air cooled engine, this is one area of the engine design you want to get right to avoid overheating issues. I’ve started to list out the fundamentals and linked to some of the fundamental references for further reading.

machining a crankshaft

Machining a Crankshaft is a daunting task and so I decided to describe my approach in a step-by-step manner. This is quite a large crankshaft for an open crank engine with a 40mm stroke. The crankshaft started life as a piece of flat 12.7mm thick flat mild steel. The design principles of the crankshaft are described on a separate post.

The Exhaust is designed to transport waste combustion gases from the exhaust valve outlet to a suitable point. The exhaust has a number of other functions such as acoustic attenuation and the primary method of achieving this is using an expansion chamber or silencer.

exhaust silencer

Exhaust Silencer – the expansion chamber of the exhaust silencer gives a broadband attenuation, with null points related to the overall length of the chamber itself. The maximum attenuation is governed by the ratio of the cross-sectional areas of the connecting pipes to that of the expansion chamber.

a cast iron flywheel

The fundamental flywheel design principles are around the inertia that is required to store energy, for an engine this is energy required to continue the rotation between firing strokes, and the minimal inertia that will then allow the engine to rev freely.

Fuel – A few pages on different fuels, starting with model engine diesel fuel.

gas turbine combustion chamber

Gas Turbines – are very quiet when compared with the conventional glow plug two and four stroke engines as used by the majority of model aircraft fliers. These engines are, however, inefficient in the terms of fuel consumption, but the fuels they do use such as petrol-diesel mixtures or propane are a lot cheaper and more readily available compared to the methanol, castor oil and nitro-methane mixes used by glow-plug engines.

gear parameters

Gear Design – Some fundamentals about gear terminology and design parameters, separate page on some of the basics with regards to gear noise.

gudgeon pin types

Gudgeon Pin – locates the piston and conrod, transfering the combustion force and at the same time allowing the parts to rotate.

History of Engines – listing of engines, features and significant events in the development of all types of engines.


Ignition System – When we talk about the engine ignition system there are two fundamental types:

  1. compression ignition
  2. spark ignition.

This is the system for igniting an fuel-air mixture that has been compressed in the cylinder by the motion of the piston.

Internal Combustion Engine – Any engine, either reciprocating or rotary, in which the fuel is consumed in the interior of the engine rather than outside of the engine. Examples of internal combustion engines are diesel and petrol engines.

steam cylinder

Insulated Cylinder – for steam engines it is important to insulate the cyclinder to reduce the condensation of steam on the cylinder wall.

Often this is just an air gap, but often this is filled with a fibre or a thin layer of wood and an outer layer of brass.

Little End – The gudgeon pin end, the part of the conrod that fits to the piston.

Loop Scavenged Engine – An external blower is used to supply the charge, under some pressure, at the inlet manifold.

Low Tension Leads – The wiring in the ignition system that is distinguished from the high tension wiring.

Main Bearings – The bearings that house the crankshaft within the cylinder block.

Model Engine Kits and Plans – a list of people around the internet and the world who sell everything from model engine plans, castings through to machined kits ready to assemble.

crankcase and bracket

Mounting Brackets – there are 10 fundamental design rules to follow to achieve the best design.

Oil & Lubrication – Thicker oil reduces surface contact of load-bearing surfaces. If the oil is much thicker than required, friction and losses will increase. Thicker oil increases losses through internal fluid friction and churning losses.

different types of piston

Pistons and Liners – there are a number of different combinations of materials that are used in model engines, some work better than others. Where we’ve used the combination I’ve included images of the parts and comments.

1/4 inch spark plug

Spark Plug – Electrical device that fits into the cylinder head of an internal-combustion engine and ignites the gas by means of an electric spark.

indicator diagram

Steam Indicator Diagram – the pressure in the cylinder is plotted versus the cylinder volume. The closed figure C D E F G H is drawn by the indicator, and is the result of one indication from one side of the piston of an engine.

This was developed by James Watt and John Southern to improve the efficiency of the steam engine. The work done is the area within the red curve.

stirling engine

Stirling Engine –  invented in 1816 by a Scotch clergyman, Robert Stirling, as a competing technology to the steam engine.

The low temperature Stirling engine is just a variant of this design where the temperature source and sink is operating at a smaller delta.

Throw of Crankshaft – The distance between the center of the crankpins and the center of the journals of the crankshaft. It is equal to half the stroke of the engine.

Top Dead Centre – The position of the crank when the piston is in its farthest position from the crankshaft, in its nearest position to the cylinder head. Abbreviated tdc.

two-stroke animation

Two-Stroke Engine – An internal-combustion engine that has one power stroke per revolution. The two-stroke spark ignition engine suffers from two big disadvantages: 1. fuel loss and 2. idling stability. The two-stroke Compression Ignition engine does not suffer from the disadvantages of the spark ignition engine which are fuel loss and idling stability and hence compression ignition engine is more suitable for two-stroke operation.

model engines

There’s a significant number of different Types of Engine: steam, internal combustion, gas turbine, stirling engine and then within those we have another breakdown in terms of different designs that have been optimised for different jobs. Also, lots of examples of engine design and model manufacture at it’s best.

Vacuum Engine – The working principle is similar to the hot air engine, except that this engine draws hot air into the cylinder. Mick Cherry has built some great Vacuum Engines.

Valves – inlet and exhaust valves: the function of the engine valves is to let air in and out of the cylinders.