Internal combustion engines and gensets: Engineering methods

Diesel backup, gas balancing plants and vehicle engines compared

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Three families share the piston engine: diesel gensets for backup and remote power, gas engines for grid balancing and data centres, and petrol and diesel engines for vehicles.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
Diesel generators¹⁴⁵	leading heavy-duty diesels reach 46-48% at the shaft; small sets at part load are much lower	2 kVA to 7,000 kVA per set in India (most 15-2,000 kVA); mtu Series 4000 at 2.8 MW	Rs 25-40 per kWh; fuel 75-80% of cost	Expensive fuel, local air pollution and noise; emission norms raised set prices by 20-40% in India.	Starts in seconds; backup sets are tested monthly, or quarterly with some new designs.
Gas engines³⁴⁵⁶	about 50% electrical for the largest	about 0.45 MW (Volvo Penta G17) to 18.9 MW per engine; plants to over 400 MW	Gas gensets Rs 13-14 per kWh in India; US reference USD 2,240/kW	Methane slip (unburnt gas); many units to maintain.	Plant minimum load about 1%; full load in under two minutes.
Petrol and diesel engines (vehicles)¹²⁹	about 28-32% from a typical petrol engine in mixed driving (calculated) and 16-25% to the wheels; 40-41% best car peak; 53.09% record truck diesel	small car engines up to heavy-truck diesels	See the road vehicle tiles for cost per km.	Light-load driving keeps average efficiency well below peak.	Mature, with very large service networks.

All three convert fuel to shaft power with similar physics, but scale and duty cycle drive the result. Large gas engines and heavy diesels run near their best point and reach about 46-53%; car engines peak at 40-41% but deliver only about 28-32% in everyday driving because they spend most time at light load. Engines beat gas turbines on part-load efficiency and start time (seconds to minutes), so they win backup and balancing roles; their weaknesses are fuel cost (diesel), local pollution and maintenance of many moving parts. Diesel gensets are among the cheapest generators to buy but among the most expensive to run.

How each method works

Diesel generators

A high-speed diesel engine drives an alternator on a common frame. Used for standby at hospitals and data centres, and as prime power on islands and off-grid sites.

Gas engines

Large spark-ignited, lean-burn engines running on natural gas or biogas, grouped into plants of many units so some can stop while others run efficiently.

Petrol and diesel engines (vehicles)

Petrol engines use spark ignition, at compression ratios of 13-14 in Toyota's latest designs; diesel engines use compression ignition and turbocharging. Hybrids run petrol engines in the efficient Atkinson cycle.

Sources

Weichai unveils diesel engine with 53.09% brake thermal efficiency DieselNet, 2024
Toyota Unveils More New Gasoline ICEs with 40% Thermal Efficiency SAE Mobility Engineering, 2018
Wärtsilä 50SG gas engine power plant Wärtsilä, 2025
Engine Power Plants Surge as Data Centers Drive Unprecedented Demand POWER Magazine, 2026
Cost Benefits: Key factors driving diesel genset uptake Power Line (India), 2024
Cost and Performance Characteristics of New Generating Technologies, Annual Energy Outlook 2023 US Energy Information Administration, 2023
FOTW #1044: 12-30% of energy put into a conventional car is used to move the car down the road US Department of Energy, Vehicle Technologies Office, 2018