Choose the engine by its rated power, not its maximum power, and keep the machine’s continuous load near 80% of that rated power. Then check the speed: the engine must deliver the rpm the machine needs, either directly on the shaft or through a belt. Finally, choose between manual and electric start based on engine size and who will operate it. This guide shows how to do each calculation, with examples for a water pump, a generator and a belt-driven machine.
Rated or maximum power: which one should you size by?
Size by rated power. A stationary engine datasheet usually gives two figures. Rated power is what the engine delivers in continuous work, hour after hour. Maximum power is about 10% higher and is only for short peaks, such as starting a heavy load.
An engine chosen by its maximum power will run at its limit all the time. It runs hotter, uses more oil and needs an overhaul sooner. The rule of thumb is simple: the machine’s continuous load should be about 80% of the engine’s rated power. The remaining 20% covers starting, load changes and normal wear over the years.
hp, PS and kW: how to convert
Manufacturers use three units, and they are not the same:
| Unit | Equal to | To convert to kW |
|---|---|---|
| PS / cv (metric horsepower) | 0.7355 kW | multiply by 0.7355 |
| hp (mechanical horsepower) | 0.7457 kW | multiply by 0.7457 |
| kW (kilowatt, SI unit) | 1 kW |
Metric and mechanical horsepower differ by just over 1%, which is too small to change the engine you choose. The common mistake is a different one: comparing an engine rated in hp with another rated in kW without converting. A 7.5 kW engine is about 10.2 PS. An engine sold as “10 hp” is about 7.5 kW.
Our datasheets give both units, and the “hp” figure on them is metric horsepower (PS).
How much power does your machine need?
Start from the power the machine takes at its shaft, in kW. If the machine’s manufacturer gives that figure, use it. If not, you can estimate it for the two most common cases.
Centrifugal water pump. The power at the pump shaft comes from this formula:
Power (kW) = flow (m³/h) × total dynamic head (m) ÷ (367 × pump efficiency)
Small centrifugal pumps are usually 50% to 70% efficient. If you do not know the figure, use 0.6. Total dynamic head adds the lift, the suction and the pipe losses; the guide on how to choose a diesel water pump shows how to work out each part.
Generator head (alternator). The alternator loses part of the power it receives at the shaft. Small alternators are about 80% efficient. To get 5 kW of continuous electrical output, the shaft needs about 6.25 kW (5 ÷ 0.8). With the engine’s 80% margin, the minimum rated power is about 7.8 kW.
Once you know the machine’s power, divide it by 0.8 to find the minimum rated power of the engine.
Speed: direct drive or belt drive
Every machine has a working speed. The engine must deliver that rpm at the machine’s shaft, and there are two ways to do it.
Direct drive. The engine shaft turns the machine at the same speed. This works when the two speeds match. The most important case is the generator: a 2-pole alternator produces 50 Hz at 3000 rpm and 60 Hz at 3600 rpm. So a generator engine needs a rated speed of 3000 rpm for 50 Hz grids and 3600 rpm for 60 Hz grids. You set this when you order; see the guide on ordering a generator configured for your country.
Pulley and belt. When the machine turns slower than the engine, the pulleys set the speed:
Machine speed = engine speed × engine pulley diameter ÷ machine pulley diameter
A 2200 rpm engine with a 100 mm pulley, driving a 400 mm pulley, turns the machine at 550 rpm. The belt lowers the speed and raises the torque by the same ratio, less small friction losses.
High speed or low speed: what changes in practice
Stationary diesel engines fall into two groups. Air-cooled engines run fast, usually at 3000 or 3600 rpm. Horizontal water-cooled engines run slower, usually between 2200 and 2600 rpm. At similar power, the two behave very differently. Two models from our catalog show this:
| HTX-AE186 (air) | HTX-R180 (water) | |
|---|---|---|
| Rated power | 5.7 kW (7.75 PS) | 5.15 kW (7 PS) |
| Rated speed | 3600 rpm | 2600 rpm |
| Maximum torque | 17.39 N·m at 2700 rpm | 21.75 N·m at 1950 rpm |
| Specific fuel consumption | 280 g/kWh | 278.8 g/kWh |
| Net weight | 48 kg | 75 kg |
The water-cooled engine has less power but more torque, and it reaches that torque at a lower speed. This helps with machines that are hard to start or have a lot of inertia, such as mills, threshers and shredders. The air-cooled engine is lighter and suits machines that run fast and move between sites, such as portable pumps and generators.
The fuel difference shows in the larger models. Our 2400 rpm water-cooled engines between 8 and 10.2 kW have a specific fuel consumption of 240 g/kWh, against 280 g/kWh for the air-cooled engines. Over many hours a day, that shows in the fuel bill. The full comparison is in the guide to air-cooled vs water-cooled diesel engines.
Manual or electric start?
Electric start is worth it when the engine is large, when it starts many times a day, or when different people operate it. It uses a 12 V battery and a starter motor, as in a car.
Manual start comes in two forms. Small air-cooled engines use a recoil rope. Horizontal water-cooled engines use a hand crank, with a decompression lever that releases compression for the first turns. Both work well on small engines, and both have one real advantage: they do not depend on a battery.
The larger the cylinder, the more effort a manual start takes. Cold weather makes it harder still, because the oil thickens. On larger engines, electric start stops being a convenience and becomes the practical way to start the machine every day. If you choose electric start at a remote site, add battery care to the routine.
Worked example: an engine for an irrigation pump
A centrifugal pump must deliver 30 m³/h against 25 m of total dynamic head. The pump is designed for 3600 rpm, and its efficiency is unknown.
- Power at the pump shaft: 30 × 25 ÷ (367 × 0.6) = 3.41 kW
- Minimum rated power of the engine: 3.41 ÷ 0.8 = 4.26 kW
- Speed: the pump runs at 3600 rpm, so a 3600 rpm engine can drive it directly, with no belt
- Result: an air-cooled engine of 4.5 kW (6.12 PS) at 3600 rpm, such as the HTX-AE182
If the site is at altitude, the engine loses power in the thinner air. In that case, apply the derating before step 4; the calculation is in the guide on why diesel engines lose power at altitude.
Where HTX fits
We manufacture air-cooled and water-cooled single-cylinder stationary diesel engines, with rated and maximum power, speed, torque and fuel consumption published on every datasheet. Compare the models in the air-cooled range and the water-cooled range. After purchase, the diesel engine maintenance schedule shows what to do at each interval.
Not sure which engine suits your machine? Tell us what the engine will drive, the power or flow it needs, its working speed and whether it is direct drive or belt drive, or message us on WhatsApp. Our engineers will reply with a recommended model within 24 business hours.