Selection of Geometric Ratios for I.C. Engines

P M V SubbaraoProfessor

Mechanical Engineering Department

Control of Micro Actions through Macro Features…..

Heat Loss Vs RBS

Frictional Loss Vs Geometry & Speed of Engine

Engine Geometric Ratios

Engine Compression Ratio

Cylinder Bore-to-Stroke Ratio

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BRBS 2Length Stroke

Bore

Kinematic Rod Ratio

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RadiusCrank

Length Rod Connecting

Extreme Limits of RBS

• The extremes to this relationship is the inertial forces origination from the piston motion.

• To achieve high power density, the engine must operate at a high engine speed (up to 18,000 rpm for the Formula 1 engine), which leads to high inertial forces that must be limited by using a large bore-to-stroke ratio.

• For applications that demand high efficiency, a small bore-to-stroke ratio is necessary and, again because of the inertial forces of the piston, requires a slower engine speed and lower power density.

• For the marine application that has a 2.5 m stroke, the engine speed is limited to 102 rpm.

The World Largest I C Engine

Despite the green hype, internal-combustion engines will keep powering vehicles for the foreseeable future.

The Latest News

• The world’s biggest engine is the Wärtsilä-Sulzer RTA96.

• It’s the largest internal combustion engine ever built by man.

• Wärtsilä-Sulzer RTA96-C is a 14-cylinder, 2-stroke turbocharged diesel engine that was specially designed to power the Emma Maersk which is owned by the Danish Maersk.

• Wärtsilä-Sulzer RTA96, the world’s biggest engine, has a weight of 2.3 million kilogrammes.

• If the weight of the average adult person is 70 kgs, this world’s biggest engine has a weight equivalent to the weight of 33,000 people.

Economies of Scale in Sea Transportation

• Maersk Lines have done the world proud by providing cheap sea transportation that is costing cents instead of a dollar per every kg weight.

• They are able to do this by using economies of scale in sea transportation.

• It is getting cheaper to ship goods from USA to China and from China to USA.

• It has now become cheaper to transport goods from China to a US port than to transport the same goods from a US port to the final destination inland of US by a truck.

Piston Speed

• For a four stroke engine:

• For Piston moves upward.

• For Piston moves downward.

• The speed of the piston

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Rod Ratio Relationships

• Short Rod is slower at BDC range and faster at TDC range.• Long Rod is faster at BDC range and slower at TDC range.

Short Connecting Rod

Effect of Rod Ratio on ISFC : PI Engine

Effect of Rod Ratio on Heat Balance : PI Engine

Creation of Constant Volume Combustion Engine

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Effect of Rod Ratio on Heat Balance

Indicative Specific Fuel Consumption

LONG ROD

• Intake Stroke -- will draw harder on cylinder head from 90o ATDC to BDC.

• Compression Stroke -- Piston travels from BDC to 90o BTDC faster than short rod.

• Goes slower from 90o BTDC to TDC--may change ignition timing requirement versus short rod as piston spends more time at top.

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Long Rod : Ignition

• If flame travel is too fast, detonation could occur.

• Does a long rod produce more efficient combustion at high RPM--measure CO, CO2?

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Long Rod : Power Stroke

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• Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20o to 75o ATDC less force is exerted on the crank pin than a shorter rod.

• The piston will be higher in the bore for any given crank angle from 90o ATDC to BDC and thus cylinder pressure could be higher.

• Long rod will spend less time from 90o ATDC to BDC--allows less time for exhaust to escape on power stroke.

Long Rod : Exhaust Stroke

• Exhaust Stroke : The piston will be more in Bore from BDC to 90o ATDC and thus cylinder pressure could be higher.

• Will force more exhaust out from BDC to 90o BTDC.

• Could have more pumping loss!

• Could be if exhaust port is poor, a long rod will help peak power.

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