How a diesel engine works: the four strokes explained

A diesel engine burns its fuel without a spark. It squeezes air in the cylinder so hard that the air gets hot enough to light diesel the moment it is sprayed in. That one idea, called compression ignition, explains most of what makes a diesel different from a petrol engine. The rest is a cycle of four piston strokes that repeats hundreds of times a minute.

What is compression ignition?

The US Department of Energy's Alternative Fuels Data Center puts it simply: diesel fuel is injected into the combustion chamber and ignited by the high temperatures reached when the piston compresses the gas. A petrol engine uses a spark plug instead.

To get the air that hot, a diesel squeezes it into a very small space. The figure that tells you how much is the compression ratio: the volume above the piston at the bottom of its travel divided by the volume left at the top. Kirloskar prints it on its engine spec sheets.

Engine (Kirloskar catalogue)	Use	Compression ratio	Fuel injection
TAF1, 6 hp, 1 cylinder	Pumpset	17.5 : 1	Direct
DAF8, 8 hp, 1 cylinder	Pumpset	17.5 : 1	Direct
DAF10, 10 hp, 1 cylinder	Pumpset	17.5 : 1	Direct
FA, 6 hp, 1 cylinder	Pumpset	19 : 1	Direct
Varsha 2 (VA320-2), 4 hp, 1 cylinder	Pumpset	22 : 1	Indirect combustion

Every engine on that list is described by Kirloskar as a "four stroke cycle, compression ignition diesel engine".

What happens in the four strokes?

A stroke is one trip of the piston, up or down the cylinder. Four strokes take two turns of the crankshaft.

1. Intake. The piston moves down and the intake valve opens. The cylinder fills with air only. There is no fuel in it yet.
2. Compression. Both valves close and the piston moves up, squeezing that air into the small space at the top. In a 17.5 : 1 engine the air ends up in about one-seventeenth of the room it started in, and it heats up sharply.
3. Power. Just as the piston nears the top, the injector sprays a fine mist of diesel into the hot air. It lights on its own. The burning gas pushes the piston down hard, and this is the only stroke that does work.
4. Exhaust. The exhaust valve opens and the piston moves up again, pushing the burnt gas out. Then the cycle starts over.

Because only one stroke in four makes power, a single-cylinder engine relies on its flywheel to keep turning through the other three. Engines with more cylinders time them so that power strokes overlap.

Worked example: inside a 6 hp pumpset engine

Kirloskar's catalogue gives these figures for the TAF1, a single-cylinder, air-cooled engine used on 6 hp pumpsets:

• Bore (cylinder width) 87.5 mm, stroke (piston travel) 110 mm
• Cubic capacity 0.662 litre
• Compression ratio 17.5 : 1
• Rated output 4.42 kW (6 hp) at 1,500 rpm
• Specific fuel consumption 195 g per hp-hour (+5%)

Here is what those numbers mean, worked out with simple arithmetic:

• Swept volume. The area of an 87.5 mm circle times 110 mm of travel is about 661 cc, which matches the 0.662 litre on the sheet.
• Space left at the top. With a ratio of 17.5 : 1, the space above the piston at the top of its stroke is 662 ÷ 16.5, or about 40 cc. So about 700 cc of air is squeezed into about 40 cc.
• How often it fires. At 1,500 rpm, with one power stroke every two turns, the cylinder fires 750 times a minute.
• Fuel per hour. 195 g × 6 hp = 1,170 g of diesel an hour at full output. At 0.845 kg a litre (the diesel density Mahindra Powerol uses on its genset sheets), that is about 1.4 litres an hour.
• Fuel per power stroke. 1,170 g spread over 45,000 power strokes an hour is about 0.026 g, or 26 milligrams, of diesel each time.

Why do bigger engines have more cylinders and a turbo?

Bigger engines split the work across several cylinders. Cummins's 6BTA5.9-G6, a genset engine, is six cylinders in a line, each with a 102 mm bore and 120 mm stroke, for 5.9 litres in all. Its sheet calls it "4 cycle, in-line, turbocharged".

At 1,800 rpm each of its six cylinders fires 900 times a minute, so the engine gets 5,400 power strokes a minute against the TAF1's 750. That is part of why it rates 119 kW on prime power against the TAF1's 4.42 kW.

The same sheet lists three parts that help a bigger engine:

• Turbocharger. A small turbine spun by the exhaust drives a fan that pushes extra air into the cylinders. Cummins says its Holset HX35 turbo gives more power, better fuel economy and less smoke and noise.
• Aftercooler. Squeezed air from the turbo is hot; cooling it packs more air into each cylinder. The 6BTA5.9-G6 is listed as "jacket water after cooled".
• High-pressure fuel pump. Its Bosch A-series in-line pump works at high injection pressure, which Cummins says gives cleaner combustion and lower emissions.

Direct or indirect injection?

In a direct injection engine the fuel is sprayed straight into the cylinder; in an indirect engine it goes first into a small chamber beside it. Spec sheets say which. Of the Kirloskar engines above, only the Varsha 2 uses indirect combustion, and it has the highest compression ratio on the list. The Cummins engine is direct injection.

What cleans up the exhaust?

Burning diesel leaves soot and oxides of nitrogen. On modern diesel vehicles, the Alternative Fuels Data Center describes an exhaust system with a particulate filter, an oxidation catalyst and a selective catalytic reduction unit, where diesel exhaust fluid turns oxides of nitrogen into nitrogen and water. In India, gensets up to 800 kW sold since 1 July 2023 must meet the CPCB IV+ emission norms, as our guide on what size genset you need explains.

What this means when you buy

• Two engines with the same horsepower can differ in rpm, cylinders and fuel use. Read the spec sheet, not just the hp figure.
• Specific fuel consumption (g per hp-hour or g per kWh) tells you how much diesel each unit of work costs, and its tolerance, such as Kirloskar's +5%. Our story on genset diesel consumption per hour turns such figures into a monthly bill.

Kirloskar Oil Engines and Cummins India are both listed engine makers; their results are on the Kirloskar Oil Engines and Cummins India pages.

Sources: US Department of Energy, Alternative Fuels Data Center, "How do diesel cars work?" (afdc.energy.gov); Kirloskar Oil Engines, Agri Business Product Catalogue, technical specifications for TAF1-NW2+, DAF8-NW2+, DAF10-NW2, FA and Varsha 2 (an older catalogue whose company figures date from 2010; current models may differ); Cummins, 6BTA5.9-G6 specification sheet (2019, power.cummins.com); Mahindra Powerol genset spec sheets (diesel density 0.845); Central Pollution Control Board, CPCB IV+ norms. Swept volume, clearance volume, firing rates and fuel per stroke are our arithmetic from these sheets.