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How Does A Diesel Engine Work?

By Featured Writer, Mike McGlothlin


The diesel engine has been around since 1897, but over the past 125-years Rudolf Diesel’s original design hasn’t change as much as one might think. Sure, diesels don’t run off of coal dust or peanut oil anymore, but they do continue to rely on compression ignition for combustion, as well as low rpm operation. In fact, in many ways the modern diesel engine works the same way it did more than a century ago. Thermal efficiency, fuel consumption, an inherently lean air/fuel ratio, no pre-detonation headaches, and abundant low rpm torque production continue to be its primary advantages. So how exactly does a diesel engine work? Using the most common form of diesel propulsion in use today—a four-stroke, turbocharged, and direct injection version—let's take a comprehensive walk through the compression ignition engine’s internal combustion process.


Four Strokes

A four-stroke diesel engine employs the same piston cycle(s) every other form of four-cycle, internal combustion engine does. First, you have the intake stroke, where the intake valve(s) opens as the piston moves to the bottom of its stroke. This is followed by the compression stroke, in which both the intake and exhaust valves are closed while the crankshaft forces the connecting rod and piston assembly up, to the top of the cylinder bore. Then comes the power stroke (or combustion stroke), where fuel is injected in the cylinder, thereby initiating combustion, and the piston is forced down again. Finally, the exhaust stroke occurs, where the exhaust valve(s) open and the piston travels upward, pushing exhaust gases out.


Compared To Gasoline

Because most consumer vehicles in North America are (and have long-been) gasoline-powered, we’ll kick things off by comparing the gasoline internal combustion engine with the diesel internal combustion engine. First, it’s important to establish that a diesel engine is very similar and yet very different from a gasoline engine. The same basic structural makeup is in play—and by that we mean a crankshaft spinning and converting downward force into rotary movement, connecting rods and pistons moving up and down within the cylinder bores, air being pumped in, and exhaust being routed out—but the means of combustion is where the two part ways.


No Spark, Higher Compression & No Air/Fuel Pre-mixing

Gasoline engines use spark plugs (i.e. spark ignition) to execute the combustion process, but in a diesel power plant there are no spark plugs or other combustion aids. Thanks to a diesel engine’s high compression ratio (which can range anywhere from 15:1 to 22:1), compressed in-cylinder air is superheated and requires only the addition of fuel to begin the combustion process, hence the term compression ignition. On top of that, no mixing of air and fuel occurs prior to the power stroke taking place within a diesel engine. In direct contrast, a port fuel injection style gasoline engine introduces a mixture of fuel and air in the cylinder on the intake stroke.


The Power Stroke

So, if no pre-mixing of air and fuel occurs before the power stroke occurs in a diesel engine, what type of fuel injection process takes place to initiate combustion, and what components are involved? A highly pressurized pulse of fuel, distributed through the nozzle of an injector positioned within the cylinder head, delivers fuel at a specific spray angle and directly into a fuel bowl that’s integrated in the piston. In direct injection diesel engines, the pressure leaving the injector nozzle can range from 10,000 psi to more than 30,000 psi. When this kind of pressure is introduced in-cylinder (which again is full of highly compressed and extremely hot air), diesel fuel ignites without the need for any external ignition source.



If you’re curious about the math behind compression ignition, the equation PV=nRT (also known as Ideal Gas Law) is invaluable. PV=nRT explains the relationship between pressures (P), volume (V), the amount of gas present as measured in moles (n), the universal gas constant (R), and temperature (T).Cutting to the chase, as pressure rises within the cylinder, temperature increases also. In a diesel engine, when the piston compresses the air above it to a fraction of its original volume, in-cylinder temperature easily exceeds 400 degrees F. This is more than enough heat and pressure to ignite highly atomized diesel fuel without the need for spark plugs.


Diesel Fuel Injectors

Mechanical, pop-off style fuel injectors, unit injectors (with either mechanical or electronic control), and electronic, high-pressure common-rail injectors are all commonly employed in direct injection diesel engines. Pop-off style fuel injectors operate 100-percent mechanically. Once a predetermined pressure is achieved within the injector body, the check valve lifts off its seat, allowing fuel to flow through the nozzle. In a unit injector, each fuel injector is essentially its own high-pressure pump capable of pressurizing its incoming supply pressure from as low as 55 psi to as high as 29,000 psi. A high-pressure common-rail injector, electronically controlled and activated via an electric solenoid, can perform five or more injection events per combustion cycle, as well as operate at peak pressures that exceed 30,000 psi.


Precision Nozzles

Diesel injector nozzles provide the precise in-cylinder fuel distribution that’s required for efficient and complete combustion. They are manufactured from high-quality materials such as low-fatigue alloy steel (often carburized for added hardening) in order to stand up to the extreme heat they’re exposed to. A typical nozzle will feature five, six, seven, or even eight orifices—all of which are precision machined via electrical discharge machining (EDM) to deliver the appropriate flow, spray pattern, and spray angle onto the piston beneath it. Once combustion has taken place, excess fuel left in the injector is routed through a return in the injector body and ultimately back to the fuel tank. Aside from unit injectors (which handle their own, internal pressurizing of fuel), where does a diesel engine’s aforementioned ultra-high injection pressure come from? The injection pump.


Inline Injection Pumps

Be it mechanical or electronic, the injection pump is the heart of the diesel engine. Not only does the injection pump provide precisely metered fuel, sufficient fuel quantity and flow, and in many cases injection timing, but it essentially acts as the engine’s throttle. Mechanical inline injection pumps, also known as jerk pumps and/or plunger pumps, are some of the oldest diesel injection pumps you’ll come across. The inline injection pump’s case (or block) houses a camshaft (driven by the engine) that controls the injection rate, plungers within barrels that compress and pressurize fuel, and delivery valves that control the overall fuel volume flowing to each injector. Each individual plunger in an inline injection pump feeds its own fuel injector, and the pump itself is timed with the engine.


Rotary Injection Pumps

Also known as a distributor type injection pump, a mechanical rotary unit is an axial-piston pump that employs a vane-style supply pump to raise internal fuel pressure, which increases as engine speed increases. The pumping plunger(s) (also known as a distributor plunger) is responsible for creating injection pressure and controlling injection timing. Electronic rotary injection pumps typically add an ECU to the mix, which brings with it more precise operation and provides for finer adjustments in fueling. Within a rotary pump, a high-pressure solenoid valve can also be utilized to control fueling independent of engine speed.


High-Pressure Fuel Pump (Common-Rail)

Often referred to as the HPFP for short, a high-pressure fuel pump is found on today’s electronically controlled diesel engines utilizing high-pressure common-rail injection. In some cases, the HPFP’s sole job (other than regulating fuel) is to create pressure—and not to control injection timing or quantity. Pumps such as the Bosch CP3, for example, operate independently of engine speed. HPFP’s are radial-piston pumps that utilize a camshaft, plungers, a fuel pressure regulator (also known as an FCA, MPROP or VCV), and are capable of producing injection pressures as high as 39,000 psi.


Fuel Is Throttled - The Air Follows

Why so much talk about fuel in regard to how a diesel engine works? Because diesel engine operation revolves around fuel and fuel alone. Contrary to a gasoline engine, where you’re throttling airflow by way of the throttle body, in a diesel engine you’re throttling fuel, and no throttle body exists. This works because the diesel engine is capable of operating on a very wide range of air/fuel ratios (as lean as 100:1 in some applications). Once you’ve provided the engine with fuel, the air follows, which brings us to the airflow side of a diesel engine.



Like any internal combustion engine, a diesel breathes through an air filter. But almost all diesel engines these days are equipped with a turbocharger, which means the clean air passing through the filter element feeds right into the compressor wheel of a turbo. There, the air is compressed and (typically) forced through either an air-to-air intercooler or an air-to-water intercooler before entering the cylinder head(s). Highly efficient heat exchangers, intercoolers provide for a cooler, denser intake charge to enter the engine—and they do so with very little boost loss between the turbo and engine. Reduced exhaust gas temperature (EGT) is another benefit of running an intercooler.


The Turbocharger

As we alluded to earlier, most diesel engines in production today are designed around the use of, and are equipped with, a turbocharger. This gift from the manufacturer means the diesel engine’s thermal efficiency is optimized over what you’ll find in any naturally aspirated internal combustion engine. And not only does the diesel engine’s turbocharger provide improved volumetric and thermal efficiency, but (as with any turbo) the waste heat from the engine’s exhaust is used to drive it (i.e. free horsepower). Exhaust gases leaving the head(s)—through either exhaust manifolds and up-pipes on V engines or a single exhaust manifold on an inline engine - spin the turbine (exhaust wheel). And because the turbine shares a common shaft with the compressor wheel, the process of producing compressed air (boost) for the engine to ingest commences uninterrupted, with continuous power production.


Direct Injection (DI)

Along with most late-model diesel engines being turbocharged, the majority of them also make use of direct injection rather than indirect injection (more on indirect injection below). In a direct injection system, fuel is injected directly into the cylinder, with the spray of fuel concentrated within the fuel bowl present in the piston. A direct injection style piston (featuring what is often referred to as the “Mexican hat” design) is specifically designed to house its own combustion chamber. However, there is more to the story than that. During the diesel engine’s compression stroke, the Mexican hat design induces rapidly swirling air. This swirl effect provides superior mixing of air and fuel once the injector fires.


Indirect Injection (IDI)

In the days of old, indirect injection (IDI) was more common than it is today, although this means of fuel delivery is still in use in many small displacement diesel engines. In an IDI engine, fuel is introduced via a pre-chamber (also known as a swirl chamber) located in the cylinder head(s). The combustion event initiates within the pre-chamber and expands into the cylinder beneath it. No fuel bowl or combustion chamber is integrated into the piston in an IDI engine, and the pistons don’t look much different than what you’ll find in a gasoline engine. However, similar to a direct injection diesel engine, immense airflow turbulence is created during the compression stroke, which results in the proper fuel/air mixture once fuel is introduced.


Grid Heater

Thanks to their high compression ratio, diesel engines require substantial cranking power to turn over. This is why you’ll find two or more batteries in most diesel applications. In cold weather, starting problems are often amplified, and for this reason starting aids such as grid heaters and glow plugs are employed. A grid heater (also known as an air intake heater) is located in the air intake tract and is located close to the cylinder head(s). It is a stand-alone onboard device that typically only calls for 12-volt power and warms the incoming air to improve engine startup. Grid heaters are also used to reduce cold-start emissions on diesel engines.


Glow Plugs

Another cold-weather starting aid often employed in diesel engines is the glow plug. Glow plugs are located in the cylinder head(s) and are positioned in close proximity to the fuel injectors. Similar to the injector nozzle, they protrude into their respective cylinder. Glow plugs are typically made from platinum or iridium to resist oxidation and withstand the high-heat conditions they’re exposed to. Either a glow plug relay or glow plug controller supplies the requisite power (via the vehicle’s onboard batteries) to operate them. In many applications, electric current is applied to the glow plugs for a specified period of time.


When A Diesel Engine Works, Hard, Big Torque Is Produced

High compression, tremendous cylinder pressure, elevated boost pressure, and an almost-obligatory long stroke crankshaft all present the perfect storm for a diesel engine’s ability to produce so much low rpm torque. On top of decreasing a diesel engine’s pumping losses on the intake stroke, the boosted intake air that comes courtesy of the turbo increases the average effective cylinder pressure on the power stroke. A quick burn of fuel and an ability to continue that burn well into the power stroke (thanks to lengthening the pulse width/injector on-time) also helps produce higher cylinder pressure—and cylinder pressure is directly linked to torque production. However, too much cylinder pressure can lead to engine failure, which is why the average diesel engine is designed so robustly.


Overbuilt On Purpose

The parts and pieces that allow a diesel engine to operate flawlessly for hundreds of thousands of miles (or tens of thousands of hours) must be extremely rigid in order to hold up to the kind of cylinder pressures they see. Deep-skirt, cast-iron (and sometimes even compacted graphite iron) crankcases, block stiffeners, cylinder liners, forged-steel crankshafts and connecting rods, large bearing surfaces, six head fasteners (or more) per cylinder, and gear-driven cams, injection pumps, power steering pumps, vacuum pumps and oil pumps are all par for the course. Induction hardening, shot-peening, micro-polishing, nitriding and various other processes for improving fatigue strength and lower operating friction are also employed at the OEM level to ensure maximum longevity of a diesel engine's hard parts.


Why The Diesel Engine’s Work Ethic Is So Hard To Beat

Thanks to burly parts, low rpm operation, and arguably the most efficient form of fuel injection on its side (direct injection), the diesel engine is the most versatile internal combustion engine ever designed. Thanks to direct injection, high-pressure fuel delivery and turbocharging, the diesel engine is fuel-efficient, clean-burning, and capable of producing more-than-adequate horsepower and torque figures—and can do it around the clock. And despite ever-tightening emissions standards being rolled out in the modern era, underneath all of the new PM, NOx and CO2-scrubbing aftertreatment components the same basic concept of what makes a diesel engine so great remains.


Engine Of The Past, Present And Future

With all the buzz about electric vehicles (EVs) and plug-in hybrid technology, the consumer automotive market may be changing at the present time. But in the transportation, construction, agriculture, military, and marine sectors—and anywhere else where hard work is the order of the day—diesel engines will continue to dominate the landscape far into the future. For more than a century, compression ignition has proven itself to be the most durable, feasible, fuel efficient, powerful, clean-burning, and readily available means of propulsion on the planet. Rest assured, diesel isn’t going anywhere anytime soon.

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