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Why Do Diesel Engines Tend To Run at Such Low RPMs?

Why Do Diesel Engines Tend To Run at Such Low RPMs?

Diesel engines have a much lower red line than gasoline engines. In fact, many diesel engines are manufactured with a red line of less than 4500 RPM, which is almost half the RPM of a gas engine. The question is, why? Why do diesel engines tend to run at such low RPMs?

Diesel engines tend to run at low RPMs because they have a longer stroke length and are heavier. Notably, the longer stroke length creates more torque at lower RPMs. Still, the additional weight means more inertia to overcome to get the engine up to speed. 

This article will discuss the various reasons for the low RPM of a diesel engine and detail the pros and cons of having low RPM. Read on for more insights into these and how to get more power out of a diesel engine.

Why Diesel Engines Have Low RPM

In a nutshell, diesel engines have low RPM due to three main factors:

  • Diesel engines have a longer stroke length.
  • Diesel engines are powered by compression.
  • Diesel engines are heavier.

I’ll describe each of these below.

Diesel Engines Have a Longer Stroke Length

Diesel engines tend to run on lower RPMs because they have an increased stroke length. To put this in perspective, the stroke length is the distance a piston travels from the top dead center (TDC) to the bottom dead center (BDC).

It seems intuitive that the greater distance, the more time it takes for the piston to travel. Therefore, the slower RPMs of a diesel engine are attributed to its increased stroke length.

For example, the Mercedes-Benz C 200 gasoline variant comes with an M274 engine with a stroke length of 73.7 – 92mm (2.90 – 3.62in). The engine delivers up to 242hp (180kW) at 5,500 RPM. This RPM is significantly higher than what the diesel variant uses.

Notably, the Mercedes-Benz C 200d’s OM651 engine comes with a stroke length of 83 – 99mm (3.27 – 3.9in) that delivers 136hp (100kW) at 2,800-4,600 RPM.

Diesel Engines Are Powered by Compression

Compression is the act of compressing air and fuel into an air-tight space before the combustion process begins. The more efficient the compression, the less chance there is for pre-ignition to occur, which can cause knocking in the engine.

The compression ratio is the difference between the largest and smallest volume in an engine. 

Diesel engines tend to have high compression ratios, making it easier for them to achieve high power output at low RPMs. This is because the diesel engine can compress more air into a small space over a given period.

Here’s a video that describes how compression ratio affects an engine’s efficiency:

5 Reasons Diesel Engines Make More Torque Than Gasoline

Diesel Engines Are Heavier

Diesel engines tend to be a bit heavier than their gasoline counterparts because diesel engines feature heavier parts. Generally, the heavier the engine, the more inertia it must overcome before reaching its maximum RPM. This causes diesel engines to have a lower redline than gasoline engines.

What RPM Is a Diesel Engine Most Efficient?

Diesel engines operate most efficiently between 1800 and 2000 RPM. At these RPMs, the engine attains its highest torque and power. Notably, this is where EGR (exhaust gas recirculation) valves can be optimized to prevent knocking and low oil pressure issues such as hydraulic lifter collapse.

In general, this range doesn’t require too much from the transmission. Below or above it, you may get lower power output and efficiency.

Is It Good To Have Low RPM?

Low RPM comes with both pros and cons. The upsides include reduced wear on parts, lower vibration and noise levels, and better fuel economy. The downsides include less propensity to achieve high speeds.

Therefore, there is a case for both high and low RPM. In many cases, the performance benefits of low RPM outweigh its downsides.

How To Get More Power From a Diesel Engine

Now that you know why diesel engines have low RPMs and the pros and cons of that, let’s look at some of the ways to get more power from these machines.

1. Get a Turbocharger

A turbocharger is a device that increases the flow rate and pressure of air entering the engine. This increased airflow results in more fuel being burned per cycle, and in turn, causes higher power output. The increased airflow also improves combustion (source).

In other words, a turbocharger helps diesel engines achieve more power from the same amount of energy. This is especially useful in cars with strict emission standards, as the turbocharger can reduce nitric oxide from being produced.

2. Get a Supercharger

A supercharger is a powered compressor that increases the airflow and pressure of the air entering the engine. This, in turn, results in more fuel being burned per cycle and an increase in torque and power output. However, it typically engages at lower RPMs than a turbocharger (source).

3. Get a New Camshaft Profile

A camshaft is a device that controls the opening and closing of valves in an engine. The specific timing of valve opening and closing determines how much air/fuel mixture enters the cylinder during the power cycle, which ultimately decides torque or power output (source).

A camshaft also controls the compression ratio of the engine, which determines how much air can be compressed into a given amount of space before combustion begins.

4. Get a New Intake Manifold

An intake manifold is a device that channels air from the air filter assembly into various cylinders in an engine. A new intake manifold can increase the airflow and pressure of air entering the cylinders. As a result, the motor will burn more fuel at any given RPM, which increases power or torque (source).

5. Never Skimp on Maintenance

You should always maintain your car, including changing belts, glow plugs, and air filters. If you’re in the market for new air filters, I recommend this K&N Universal Air Filter from Amazon.com. It’s easy to install, reusable, and cost-effective.

Also, you should clean and replace fuel filters regularly, besides cleaning your unit’s EGR valves regularly. 

Keeping up with these basic maintenance tasks ensures that you get the most power output from your diesel engine. Otherwise, neglecting them will lead to lower power output, along with accelerated wear and tear.

The Bottom Line

In summary, diesel engines run at low RPMs because of their longer stroke length, heavier weight, and higher compression ratio. As a result, diesel engines suffer from less wear and tear as a whole. They also tend to be quieter at lower RPMs due to the decreased vibration levels.

Still, there are different ways to increase power output from a diesel engine. These include turbocharging, supercharging, replacing camshafts, intake manifolds, and always maintaining your car.

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