Why trucks use diesel engines: technical and economic advantages

You’ve probably wondered why trucks and heavy-duty vehicles are equipped almost exclusively with diesel engines, while small cars predominantly use gasoline. The explanation is not just about tradition or preference, but about fundamental technical differences and economic considerations that make diesel power the ideal choice for heavy transport. The diesel engine was designed from the outset for industrial applications and heavy-duty vehicles where reliability, durability, and efficiency are priorities.

Although there are small cars with diesel engines as well, they are fundamentally different in size and configuration from the units installed on trucks. Nevertheless, the operating principle remains the same: compression of air at high temperatures that ignites the fuel by auto-ignition, without the need for spark plugs. Let us explore the technical and economic reasons why diesel dominates the commercial-vehicle segment.

High torque at low RPM

The most important technical advantage of diesel engines for trucks is the ability to produce high torque at low RPM. Torque means rotational force—the power the engine transmits to the wheels to move the vehicle. A loaded truck can weigh 40 tons or more, requiring immense force to get moving.

Diesel engines generate maximum torque between 1,200-1,800 rpm, compared with gasoline engines that peak at 3,000-4,500 rpm. This characteristic is crucial for:

• Starting with a heavy load: A truck must be able to start from rest even on inclines, with the trailer loaded
• Steady traction: At low driving speeds (50-80 km/h), the engine operates efficiently
• Lower consumption: Low revs mean fewer piston cycles, hence less fuel burned

Diesel fuel burns more slowly than gasoline due to its higher energy density (roughly 15% more energy per liter). This slower, controlled combustion allows more efficient energy extraction, converting it into usable torque. A modern truck diesel engine can generate 2,000-3,000 Nm of torque, enough to pull extremely heavy loads.

Superior efficiency and reduced consumption

Diesel engines are 20-40% more efficient than their gasoline counterparts, a crucial advantage for vehicles that cover hundreds of thousands of kilometers annually. This efficiency arises from several technical factors:

• High compression ratio: Diesel engines compress air from about 14:1 up to 25:1, versus 8:1-12:1 for gasoline. Higher compression means higher temperatures and more complete combustion of fuel.

• More efficient combustion: Direct injection of diesel fuel into the combustion chamber, precisely when compression peaks, ensures nearly complete combustion. There are no evaporative losses as with gasoline.

• Lean-burn operation: Diesel engines run with excess air (air-fuel ratio of 18:1 or higher), unlike gasoline engines which require a stoichiometric 14.7:1. The excess air ensures complete burning.

For a truck that travels 150,000 km per year, the difference in fuel consumption can mean tens of thousands of euros in annual savings. A diesel truck typically consumes 25-35 L/100 km, while a gasoline equivalent would use 30-40% more.

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Exceptional durability and reliability

Diesel engines are built to withstand hundreds of thousands or even millions of kilometers. This longevity comes from:

• Robust construction: The engine block, cylinder head, and all internal components are oversized to withstand high pressures and temperatures. Cylinder walls are thicker, the crankshaft is heavier, and the bearings are sized for extreme loads.

• Superior lubrication: Diesel fuel has natural lubricating properties, protecting fuel-injection system components. Also, engine oil is changed less often (50,000-100,000 km on modern trucks), but with superior specifications.

• Absence of ignition system: Without spark plugs, coils, and distributors, there are fewer components that can fail. Modern injection systems are highly reliable.

Modern trucks can cover 1-1.5 million kilometers before the first major engine overhaul, and some exceed 2 million km with proper maintenance. This durability helps offset the higher initial cost.

Long-term economic viability

Although a diesel-powered truck is more expensive to purchase than a gasoline one (the difference can be €15,000-€30,000), life-cycle economics clearly favor diesel:

• Fuel costs: At 150,000 km/year and a difference of 5 liters/100 km, annual savings can exceed €10,000, offsetting the price difference in 2-3 years.

• Maintenance costs: Although repairs are more complex, they are less frequent. The absence of an ignition system eliminates recurring costs for spark plugs, coils, and wires.

• Residual value: Diesel trucks retain value much better. After 5-7 years, a diesel truck can be worth 20-30% more than a gasoline-equivalent.

• Availability: The service network for diesel truck engines is far more developed, with specialized mechanics and parts readily available.

Managing vibrations and noise

Diesel engines generate more intense vibrations and noise than gasoline engines, due to the high compression ratio and explosive combustion. Yet, on a truck this disadvantage becomes negligible:

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• Vehicle mass: A 20-40 ton truck absorbs and dissipates engine vibrations far more effectively than a 1.5 ton passenger car. The cab is isolated from the chassis by massive mounts.

• Suspension mounting: The engine sits on hydraulic mounts that absorb 80-90% of vibrations before they reach the cab.

• Advanced sound insulation: Modern cabin enclosures feature multiple layers of sound insulation, reducing noise to levels comparable with passenger cars.

• Adaptation: Professional drivers quickly become accustomed to the characteristic vibrations, which can even become an indicator of proper engine operation.

Modern truck diesel engines use technologies such as common-rail injection at pressures of 2,000-2,500 bar, piezoelectric injectors, and post-treatment systems that dramatically reduce noise and vibrations compared to earlier generations.

Performance in challenging conditions

Diesel engines excel in operating conditions typical for commercial vehicles:

• Cold starting: Preheating systems (glow plugs) provide reliable starts down to -30°C. Modern engines have sensors that optimize the process.

• Extended idle operation: A truck can sit with the engine running for hours for climate control or to power equipment, without overheating issues.

• Altitude: Unlike naturally aspirated gasoline engines, turbocharged diesels maintain performance at high altitudes, the turbine compensating for thinner air.

• Constant-speed operation: Trucks run for hours at a constant engine speed (1,400-1,600 rpm), exactly the regime optimal for diesel engines.

Differences between car diesel and truck diesel

It is important to understand that diesel engines in cars and those in trucks, while operating on similar principles, are fundamentally different:

• Size and configuration: Truck engines have displacements of 8-16 liters (vs 1.5-3.0 liters in cars), with inline or V configurations of 6-8 cylinders.

• Injection pressure: Trucks use systems with pressures over 2,000 bar; cars rarely exceed 2,500 bar.

• Cooling system: Truck engines have massive radiators and cooling systems sized for continuous operation at maximum load.

• Turbochargers: Trucks use large, variable-geometry turbochargers, often with intercooler aftercoolers.

• Emissions treatment: Truck aftertreatment systems (DPF, SCR with AdBlue) are much more complex, handling large volumes of exhaust gas.

Conclusion

Diesel engines dominate the heavy commercial vehicle segment due to a unique combination of technical and economic advantages. High torque at low RPM, superior efficiency, exceptional durability, and reduced operating costs make diesel the natural choice for trucks and heavy-duty vehicles.

Although technology is evolving and alternatives are emerging (electric trucks, natural gas or hydrogen), diesel engines will remain dominant in road transport for many years to come. The existing infrastructure, proven reliability, and favorable performance-to-cost ratio are hard to match, especially for long distances and heavy loads.

For any transport company, choosing a diesel truck is not just a preference but a rational economic decision based on decades of experience and proven technical advantages.