Diesel engines have been around for over 100 years and are known for their fuel efficiency and high torque output. However, they also come with some drawbacks compared to gasoline engines. Here are the main disadvantages of diesel engines:
Higher Upfront Costs
Diesel engines typically cost more than comparable gasoline engines, often thousands more for passenger vehicles. The engine itself is more complex and built to withstand higher compression ratios and temperatures. Supporting components like the fuel injection system and turbocharger add to the costs as well. This makes diesel vehicles more expensive to purchase upfront.
Lower Power and Performance
Most diesel engines produce less horsepower and accelerate slower than gasoline engines. This is especially noticeable in smaller passenger cars. Diesel fuel also contains less energy per gallon than gasoline, further reducing performance. The torque and low-end power of diesel engines helps compensate for this, but drivers looking for fast acceleration will be disappointed.
Heavier Weight
The heavy-duty components required for diesels, like the cylinder block, add extra weight. This diminishes handling and increases wear on suspension and braking components. Reduced payload capacity is another downside of the increased weight. However, the fuel efficiency gains offset the weight penalty for many commercial applications.
More Noise and Vibration
Diesel engines are noisier at idle and throughout the rev range compared to gas engines. The combustion process causes more vibration as well. Noise insulation helps reduce cabin noise in vehicles but comes at a cost. Many buyers find gasoline engines provide a more pleasant driving experience.
Lower Resale Value
Diesels lose value faster than comparable gas vehicles. This mainly results from higher maintenance costs down the road leading buyers to offer less money. Plus, diesels make up a small percentage of the passenger car market, limiting demand. Owners need to plan on keeping their diesel vehicle longer to make up for the resale hit.
Maintenance Requirements
The advanced diesel technology requires more frequent service and specialized mechanics. Oil changes, for example, must adhere to strict intervals to avoid engine damage. Items like diesel particulate filters and DEF systems require maintenance too. Repair costs for major components are also significantly higher, especially on newer common rail diesel engines.
Slow Cold Weather Starts
Diesel engines typically have more difficulty starting in cold weather. The engines lack spark plugs and rely solely on compression for ignition. Batteries, glow plugs, and winterized fuel help, but cold starts still take longer. This problem is exacerbated at high altitudes and in northern climates.
Emissions Compliance Complexity
Meeting the latest emissions standards requires complex and expensive exhaust aftertreatment systems. Components like diesel particulate filters, diesel oxidation catalysts, EGR coolers, and DEF injection systems increase costs and maintenance needs. This complexity will only grow as emissions regulations tighten globally.
Fuel Availability Limitations
While diesel is common for commercial use, passenger car diesel can be hard to find in some regions. Traveling long distances may require planning ahead to locate stations with diesel pumps. Fleet and heavy-duty vehicles have fewer issues since truck stops stock ample supplies. But accessibility is still inferior to gasoline’s ubiquity.
Driving Style Adaptation
Drivers accustomed to gasoline engines must learn to adapt to maximize diesel efficiency. Simple things like easy acceleration from stops and maintaining consistent highway speeds help. But the power band and driving characteristics require an adjustment period for many motorists. Becoming comfortable with the different low-end torque takes time.
Potential Oil Dilution
Modern diesel engines with direct fuel injection designs can suffer oil dilution issues. Small amounts of diesel fuel make it past the piston rings into the crankcase oil. This lowers the oil’s lubricity and requires more frequent changes. It mainly affects engines driven frequently for short trips. Extended highway driving helps remove the accumulation.
Longevity Concerns
While diesel engines have a reputation for longevity, modern high-power designs may require earlier rebuilds. Tighter emissions restrictions, higher injection pressures, and increased combustion temperatures add stress. Many engines now need overhauls around 300,000 miles instead of 500,000 to 1 million miles. Of course, proper maintenance extends the lifespan.
Diesel Engine Pros
It’s important to weigh the disadvantages of diesel against the benefits. Here are some of the key advantages that make diesel engines so popular for commercial use:
Better Fuel Efficiency
Diesels achieve 10-20% better miles per gallon compared to similar gas engines. This advantage widens at steady highway speeds thanks to minimal throttling losses. The high compression ratio and lean burn improve efficiency across operating ranges. Better fuel economy lowers operating costs substantially for high-mileage fleets.
Greater Durability
Robust construction allows diesels to handle higher compression ratios, combustion pressures, and temperatures. When combined with conservative tuning, this provides outstanding durability and reliability. Fleet operators minimize downtime with diesels proven to easily cover hundreds of thousands of miles before rebuild.
Lower Emissions
Modern diesels burn cleaner thanks to scrubbing exhaust aftertreatment components. They emit less hydrocarbons and carbon monoxide than gasoline engines. Diesel particulate filters capture soot as well. While oxides of nitrogen remain higher, diesels meet the latest emissions standards when equipped properly.
High Torque Output
Diesels generate torque at lower engine speeds compared to gas engines. This allows easy lugging of heavy loads without frequent downshifting. Trucks can maintain speed on steep grades as well. Drivers appreciate the low-end power when hauling or towing heavy payloads.
Reduced Fire Risk
Since they lack spark plug ignition, diesel engines present almost no fire hazard. This makes diesels preferred for underground mining, agriculture, forestry, and heavy construction use. Gasoline poses a greater risk for sparking and ignition in these hazardous environments.
Lower Refueling Costs
Though pricing fluctuates, diesel fuel historically costs less per gallon than gasoline. Combined with better fuel economy, diesel vehicles save money at the pump despite having higher fuel tank capacities. Fleet owners enjoy considerable reductions in fuel outlays from choosing diesel power.
High Resale Value
In the commercial truck and equipment segments, diesel engines retain resale value very well. The reliability, durability, and fuel efficiency keep demand strong in secondhand markets. Fleet owners can count on recouping their original diesel investment at trade-in time.
Diesel Usage by Industry
Different industries choose diesel engines for varied reasons matching their performance to application needs. Here are some of the most prominent users:
Trucking
Diesel dominates the trucking industry with nearly 100% penetration in Class 7 and 8 tractors. The fuel efficiency translates to lower operating costs and more profit for fleets. High torque suits the frequent hauling of heavy loads. Durability keeps trucks in service for years covering hundreds of thousands of miles.
Rail
Railroads embraced diesel locomotives starting in the 1930s. Their ruggedness and high torque output fit perfectly for pulling long freight and passenger trains. Rail transport relies on diesels to cover mountainous terrain as well. Diesel-electrics remain the standard for locomotives today.
Marine
Diesel engines power the vast majority of ships from cruise liners to cargo vessels to tug boats. Their fuel efficiency makes cost-effective propulsion over long distances possible. Slow-speed two-stroke diesels running on cheaper residual fuels dominate in oceangoing vessels. Fast four-stroke diesels excel at high power in smaller craft.
Mining
Haul trucks, excavators, blast drills, and other mining equipment frequently use diesel engines. Their longevity keeps downtime low despite long hours and dirty operating conditions. Diesels also avoid the fire risk associated with gasoline engines in underground mines.
Agriculture
Tractors, combines, sprayers, and irrigation pumps rely heavily on diesel power. Farmers choose diesels for the fuel efficiency, high torque, and durability. They rack up hours in dusty, dirty environments often at low speeds. Diesels handle these conditions while minimizing operational costs.
Construction
All types of heavy machinery used in construction—dozers, loaders, excavators, cranes, graders, rollers, and more—predominantly use diesel engines. Low-end power suits the tough terrain, frequent load cycling, and heavy attachments. Durability reduces downtime common on job sites.
Emergency Services
Many ambulances, fire trucks, and police patrol vehicles utilize diesel power. The torquey acceleration provides quick response times even under heavy loads. Durability handles the demanding driving cycles as vehicles accumulate high annual miles. Diesel fuel is readily available at emergency scenes when required.
Military
Diesel engines power tanks, transport and supply trucks, bulldozers, generators and various other military vehicles and equipment. Their durability and fuel efficiency provide reliable operation in extreme conditions. Diesels even see limited use in naval vessels, submarines and some aircraft when appropriate.
Diesel Engine Types
There are three main diesel engine configurations used in the various applications discussed above:
Two-Stroke Diesel
Two-stroke diesels complete the intake, compression, combustion and exhaust process over one revolution of the crankshaft. This provides high power density, but two-stroke designs are bulkier, louder and emit more pollution. Marine applications still commonly use large two-stroke diesels providing extreme torque.
Four-Stroke Diesel
The more refined four-stroke cycle splits the combustion events over two crankshaft revolutions. Intake, compression, power and exhaust each get a dedicated stroke. This type dominates in trucks and other vehicle uses. Four-stroke diesels run smoother but are often heavier than comparable two-strokes.
Diesel-Electric
These systems consist of a large diesel engine running generators to produce electricity. Motors use this electric power for propulsion or other work. Locomotives, ships, heavy mining trucks and even some road vehicles use diesel-electric powertrains. They provide tremendous torque with a more flexible powertrain layout.
Diesel Fuel Types
While gasoline comes predominantly as one fuel grade, there are many forms of diesel fuel suited to different applications:
Highway Diesel – #2
This light diesel works well in fast four-stroke engines like those in trucks and cars. #2 diesel flows easily for reliable fueling in cold weather. It provides good energy density for peaked horsepower performance. Ultra-low sulfur formulations (ULSD) support emissions systems.
Off-road Diesel – #4
Heavier fuels like #4 diesel suit large two-stroke engines in marine and rail uses. The lower volatility prevents vaporization issues in huge combustion chambers. However, #4 must be preheated for cold weather flowability. This grade contains more sulfur as well.
Biodiesel Blends
Blends like B5 and B20 mix biodiesel made from plant oils and waste grease with conventional diesel fuel. Biodiesel burns clean but may require engine adjustments. It works better when blended with ULSD. These greener fuels are growing in popularity.
Synthetic Diesel
Lab-produced synthetic diesel provides very consistent combustion qualities. The ultra-pure composition lacks sulfur and aromatics found in conventional diesel. Synthetics prove useful for cold climate performance. Availability is still quite limited however.
Dimethyl Ether
DME represents another synthetic diesel alternative that can be made from natural gas, biomass or waste materials. It features high cetane and oxygen content for smooth combustion. DME still remains in the research and development stage so far.
Diesel Emissions and Health Concerns
One downside of diesel engines is their emissions profile, particularly older technology variants. Here are the key emissions of concern and their associated health impacts:
Particulate Matter (Soot and Smoke)
Incompletely burned fuel and engine oil produce particulate emissions from diesels. These contribute to respiratory issues and lung disease with prolonged exposure. Modern diesel particulate filters capture over 90% of particulates.
Oxides of Nitrogen (NOx)
NOx forms from high temperature combustion and contributes to smog formation. It can cause respiratory inflammation and aggravate asthma with exposure. Selective catalytic reduction (SCR) systems help minimize NOx output from current diesels.
Hydrocarbons (HC)
Inefficient, incomplete combustion allows some fuel to escape unchanged as HC emissions. These can be carcinogenic compounds like benzene, though modern diesels emit far less than older designs. Oxidation catalysts help clean up excess HC in the exhaust.
Carbon Monoxide (CO)
Diesels produce far less CO than gasoline engines since they burn fuel leanly. CO reduces oxygen delivery throughout the body and is especially harmful to those with heart disease. Diesels meet stringent CO standards.
Carbon Dioxide (CO2)
Like any engine burning fossil fuels, diesels emit CO2 proportional to their fuel consumption. CO2 contributes to climate change and environmental concerns. However, newer diesels continue improving their CO2 efficiency.
Emissions Reduction Strategies
Manufacturers utilize various methods to cut harmful emissions from modern diesel engines:
Diesel Particulate Filters (DPF)
Ceramic wall-flow filters trap over 90% of particulate matter (PM) emissions. DPFs provide passive regeneration to regularly burn off soot while driving. Filters require maintenance and eventual replacement.
Diesel Oxidation Catalyst (DOC)
A DOC speeds up chemical reactions to further oxidize HC and CO emissions into less harmful compounds. These catalytic converters are positioned early in the exhaust system right after the engine.
Selective Catalytic Reduction (SCR)
SCR systems inject a urea-based fluid to reduce NOx into nitrogen and water in a catalytic converter. DEF fluid tanks require refilling but the process slashes NOx output. SCR is very effective but adds complexity.
Exhaust Gas Recirculation (EGR)
EGR loops a portion of the exhaust gas back into the intake air. This inert gas dilutes the mixture and lowers peak combustion temperatures, reducing NOx formation. Cooled EGR improves the process further.
Lean NOx Traps
Lean NOx traps adsorb NOx under normal lean diesel operation. Periodic regeneration under rich conditions releases the stored NOx for catalytic reduction. These systems are less common than SCR but also viable.
Alternative Fuels
Renewable diesel, biodiesel, and other alt fuels burn cleaner in diesel engines. Waste greases, biomass feedstocks, and processes like Fischer-Tropsch offer reduced emissions. Blends work well in most applications.
Future Diesel Engine Technologies
Further innovations look to improve diesel emissions and performance for the future:
48V Mild Hybrid Diesel
A 48-volt integrated starter generator allows stop/start, regenerative braking and electric assist. This reduces fuel use while enabling a smaller engine. Mild hybrid diesels are entering production, led by European automakers.
Plug-in Hybrid Diesel
Combining a diesel engine with a larger electric drive system and battery pack provides PHEV capabilities. These machines operate all-electric for shorter trips and employ the diesel range extender when needed. Fuel savings can be significant.
Hydrogen Fuel Cell Diesel
Instead of a conventional starter battery, hydrogen fuel cells may provide electric power for starting, accessories and hybrid propulsion. This eliminates nearly all emissions, though infrastructure poses challenges. Applications remain limited currently.
Waste Heat Recovery
Converting wasted exhaust heat into usable energy improves efficiency. Thermal electric generators, turbo-compounding, and steam bottoming cycles are methods under development. Capturing waste heat lowers diesel fuel consumption.
Homogeneous Charge Compression Ignition
HCCI injects fuel early for a homogenous lean mixture that auto-ignites at once. This provides diesel-like efficiency with lower emissions. But combustion control, power density and emissions remain development hurdles.
Advanced Biofuels
Second generation biofuels derived from non-food feedstocks like agricultural waste, algae and other biomass show promise. These can supplant an increasing portion of petroleum diesel for fewer greenhouse gas emissions.
Conclusion
Modern diesel engines provide tremendous fuel efficiency, torque delivery, and durability. However, they come saddled with higher purchase prices, maintenance demands, emissions and noise. Extensive emissions controls help clean up diesel exhaust but add complexity. Still, diesel remains integral for the trucking, rail, marine, mining, construction and agriculture industries thanks to its benefits. Ongoing incremental innovations and alternative fuels look to enhance diesel engines further in the future by addressing cost, emissions and performance.
