Since the 1970s energy crisis, demand for higher fuel efficiency has resulted in most major automakers, at some point, offering diesel-powered models, even in very small cars.

In this interval the pressure remains constant since the piston descends, and the volume increases; the temperature rises as a consequence of the energy of combustion.

Due to incomplete combustion,[153] diesel engine exhaust gases include carbon monoxide, hydrocarbons, particulate matter, and nitrogen oxides pollutants.

[156] Helmut Tschöke argues that particulate matter emitted from motor vehicles has negative impacts on human health.

Mechanical governors are usually driven by the engine's accessory belt or a gear-drive system[160][161] and use a combination of springs and weights to control fuel delivery relative to both load and speed.

This is due to several factors, such as the two-stroke design's narrow powerband which is not particularly suitable for automotive use and the necessity for complicated and expensive built-in lubrication systems and scavenging measures.

When the piston approaches bottom dead centre, both the intake and the exhaust ports are "open", which means that there is atmospheric pressure inside the cylinder.

Uniflow scavenging is more complicated to make but allows the highest fuel efficiency; since the early 1980s, manufacturers such as MAN and Sulzer have switched to this system.

[183] Common rail (CR) direct injection systems do not have the fuel metering, pressure-raising and delivery functions in a single unit, as in the case of a Bosch distributor-type pump, for example.

This system also allows for a smoother, quieter running engine, and because fuel mixing is assisted by turbulence, injector pressures can be lower.

Activating the decompression lever locks the outlet valves in a slight down position, resulting in the engine not having any compression and thus allowing for turning the crankshaft over with significantly less resistance.

When the crankshaft reaches a higher speed, flipping the decompression lever back into its normal position will abruptly re-activate the outlet valves, resulting in compression − the flywheel's mass moment of inertia then starts the engine.

[198] Diesel engines are well suited for forced induction setups due to their operating principle which is characterised by wide ignition limits[143] and the absence of fuel during the compression stroke.

Because only air is inducted into the cylinder in a diesel engine, the compression ratio can be much higher as there is no risk of pre-ignition provided the injection process is accurately timed.

Therefore, such engines also tolerate fuels with poor ignition delay characteristics, and, in general, they can operate on petrol rated 86 RON.

[214] Smooth operation as well as high low-end torque are deemed important for passenger cars and small commercial vehicles.

The introduction of electronically controlled fuel injection significantly improved the smooth torque generation, and starting in the early 1990s, car manufacturers began offering their high-end luxury vehicles with diesel engines.

[218] Diesel engines for locomotives are built for continuous operation between refuelings and may need to be designed to use poor quality fuel in some circumstances.

In the 1940s, road vehicle diesel engines with power outputs of 150–200 metric horsepower (110–150 kW; 150–200 hp) were considered reasonable for DMUs.

Usually, the engine was mated with a pneumatically operated mechanical gearbox, due to the low size, mass, and production costs of this design.

[229][230] Additional advantages cited, at the time, included a lower risk of post-crash fire, and superior performance at high altitudes.

[232] However, noxious exhaust fumes, cold-start and vibration problems, engine structural failures, the death of its developer, and the industrial economic contraction of the Great Depression, combined to kill the program.

"[233] However, with the 1970s energy crisis and environmental movement, and resulting pressures for greater fuel economy, reduced carbon and lead in the atmosphere, and other issues, there was a resurgence of interest in diesel engines for aircraft.

[8][9][239] The United States' Federal Aviation Administration has reported that "by 2007, various jet-fueled piston aircraft had logged well over 600,000 hours of service".

[237] In early 2019, AOPA reported that a diesel engine model for general aviation aircraft is "approaching the finish line.

"[240] By late 2022, Continental was reporting that its "Jet-A" fueled engines had exceeded "2,000... in operation today," with over "9 million hours," and were being "specified by major OEMs" for Cessna, Piper, Diamond, Mooney, Tecnam, Glasair and Robin aircraft.

[239] In recent years (2016), diesel engines have also found use in unmanned aircraft (UAV), due to their reliability, durability, and low fuel consumption.

Fuel efficiency, reliability and ease of maintenance are very important for such engines, whilst high power output and quiet operation are negligible.

[244] A special class of prototype internal combustion piston engines has been developed over several decades with the goal of improving efficiency by reducing heat loss.

[250] In mid-2010s literature, main development goals for future diesel engines are described as improvements of exhaust emissions, reduction of fuel consumption, and increase of lifespan (2014).