Modern diesel engines are more advanced than the early-generation models used on trucks and tractors.
The distinctive clatter those engines made, the heavier weight, and the lack of horsepower made them uncompetitive with gas-powered engines; however, the latest models have none of the disadvantages and, with turbos attached, are sometimes superior to other engine formats.
Most modern diesel engines have turbochargers installed. The combustion process requires heavier components. The fact that they have a long piston stroke means that in their naturally aspirated (non-turbocharged) form, without turbos, they cannot compete with a similarly sized gas-powered engine.
Up until a few years ago, diesel engine technology was showing potential to compete with gas-powered engines.
They achieve better consumption figures, but the low-end torque generated is very useful in town driving or for towing. Unfortunately, they produce more pollutants, making the future of diesel much less assured.
Most Diesel Engines Have Turbos
The diesel combustion process requires more air per unit of power than gasoline engines.
For the reasons listed below, diesel engines produce less power in their naturally aspirated (non-turbocharged) form than the equivalent gas-powered engine.
Diesel Engines Are Heavier Than Gas Engines
Because the engines operate at much higher compressions than gas engines, they are designed to be more robust with heavier components.
This increase in weight results in a normal diesel engine having a low power-to-weight ratio compared to an equivalently sized gas engine.
The benefit of being heavier and using more robust materials results in diesel engines earning the reputation of being reliable and lasting for a long time.
The Ignition Process Is More Complex Than Gas Engines
The ignition process in a diesel engine happens over a shorter time, and the timing and injection process is more restricted than in conventional gas-powered engines.
Because they have a higher compression ratio (needed for ignition), the pistons in diesel engines have a longer stroke, reducing engine revolutions. This factor has positive and negative consequences.
- The longer stroke means higher levels of torque the engine produces, resulting in better acceleration from a standing start while also making diesel engine vehicles more capable as towing cars than the equivalent gas-powered car.
- The longer stroke (height of the piston chamber) limits the peak engine revs, which, in turn, reduces the ultimate horsepower of the engines.
The Solution Is Turbocharging The Diesel Engines
Engineers have two options to increase the power output of diesel engines.
Increase The Engine Size
The first possibility is to increase the size of the engine by using a bigger block with a larger bore and pistons, which is not feasible for two reasons.
- Increasing the engine size will make it even heavier.
- Increasing the size is not practical because the engine still has to fit in the same size of engine bays as gas engines.
Use Technology To Increase The Power Output
Several technologies are applied; however, the one that produces the single biggest improvement is the installation of one or more turbochargers.
Fortunately, diesel engines lend themselves to being turbocharged because the design and materials used, make them able to run at higher pressure rations.
The Benefits Of Turbocharging
By installing turbochargers on diesel engines, the engineers have improved the engine characteristics in several ways.
- Adding turbos increases the “top-end” horsepower which is available.
- When two turbochargers are installed (one activates at low revs and the other at high revs), the engine’s responsiveness is improved.
While improving the horsepower characteristics of the engine, it has not reduced the “low down” torque. If two identical cars are racing against each other, and one has a diesel, and the other has a similarly sized gas-powered engine, the following will probably occur.
- The diesel will be the quickest of the mark and accelerate faster than the gas-powered engine.
- Without a turbocharger, the diesel engine car will soon be overtaken by the other car.
- With a turbocharger, the diesel engine car will stay ahead for longer.
- The gas-powered engine would generally have a higher top speed but by a much smaller margin.
The Differences Between A Diesel And Gas Powered Engine
The two engines look similar from the outside, which makes sense because they are both categorized as internal combustion engines, which means they both require some form of an explosion to expand air and push the pistons down, which turns the crankshaft.
The difference between the two technologies is how the “explosion” is created.
The similarities and differences are listed below.
| Diesel Engine | Gas Powered Engine | |
|---|---|---|
| Process | Internal combustion | Internal Combustion |
| Cylinder Block | Yes | Yes |
| Valves | Yes | Yes |
| Pistons | Yes | Yes |
| Conrods | Yes | Yes |
| Crankshaft | Yes | Yes |
| Type of ignition | Spark Plug Ignition | Compression-Ignition |
| High Voltage ignition | High Voltage – (+20,000 volts) generated by a coil (or coil pack) | No |
| Compression ignition | No | The diesel engine initiates combustion by compressing the air. Diesel engines compress the air to a much higher degree than gas engines. |
| Method of ignition | The spark plug ignites the gas/air mixture in the compression chamber | As the air is compressed, it heats, and as the piston reaches the top dead center in the chamber, The heat reaches a point where it ignites the diesel |
| How pressure is created to push the piston down | Gasses expand outwards, creating pressure that forces the pistons down | Gasses expand outwards, creating pressure that forces the pistons down |
| Type of fuel injection | Direct or port-induced injection | Direct injection only |
| Maximum engine revolutions | 6,000 – 9,000 RPM (Formula One engines rev to 16,000 RPM) | 4,500 – 5,000 RPM |
| Main Engine Characteristic | High Horsepower | High Torque |
| Thermal Efficiency – the amount of energy that is turned into useful work | 20% – 38% (less efficient than diesel) | 40% – 45% (more efficient than gas-powered engines |
| The air/fuel mixture is rigidly controlled by the fuel injection system | There is always more fuel in a diesel engine compression chamber than in a gas-powered engine |
What Is A Turbo And How Does It Work?
In days gone by when gas-powered engines used carburetors, there would be a noticeable difference in power at sea level compared to higher altitudes.
The difference was so marked that if the car was being permanently relocated from one altitude to the other different gas/air jets would need to be installed to compensate.
With fuel injection and turbochargers, this is largely a thing of the past.
A turbocharger is a device that takes air from the engine’s exhaust and uses it to turn a turbine which sucks in the fresh air. The turbine compresses the fresh air and forces it into the air intake manifold.
This produces higher air pressure in the combustion chamber. When mixed with the correct quantity of diesel fuel, a more powerful “explosion” forces the piston down faster and increases the horsepower.
A secondary benefit is that the pressurized air produced by the turbocharger is not affected by altitude.
Conclusion
Most diesel engines have a turbocharger installed because it makes the engine more useable and directly increases the horsepower produced.
The benefit of this is that manufacturers have been able to replace gas-powered engines with more fuel-efficient diesel engines. The downside of diesel engines is that they produce more pollutants.