Diesel Engine Myths

Diesel engines are dirty and pollute more than gasoline engines

Misleading and/or requires additional context. This is not a universal truth, but indeed diesels have not historically been the cleanest running machines. However, there's a few different approaches to clarifying and, in some instances, debunking this statement.

According the United States Environmental Protection Agency (EPA), diesel vehicles produces on average 22.4 lbs of carbon dioxide (CO2) per gallon of fuel consumed, whereas gasoline vehicles produce on average 19.6 lbs of CO2 per gallon of gasoline consumed. Based on this assumption, a diesel powered vehicle needs to achieve 12.5% greater fuel efficiency than a comparable gasoline engine to emit the same levels of CO2.

Fuel Economy, Gas vs. Diesel, Various Applications

Referencing the chart above, there are many instances where the fuel economy of a diesel vehicle is significantly greater than that of its gas powered counterpart, and in each of these instances CO2 emissions are also considerably lower despite diesel fuel's higher specific CO2 emissions. This data becomes even more skewed in favor of diesel if we consider a "comparable" gasoline engine to be one of equal output rather than one of equal displacement. Therefore, diesel engines typically compensate for there higher CO2 emissions with improved fuel economy.

Another important exhaust byproduct is NOx emissions. Nitrous oxides are present in both diesel and gas exhaust streams, but are historically more concentrated in diesel exhaust because they don't tend to operate at a strict air-to-fuel ratio like gas engines. Modern diesels have adopted various technologies, including exhaust gas recirculation and selective catalytic reduction systems to dampen NOx emissions. As of current, all manufacturers of diesel powered vehicles employ SCR systems requiring the use of a diesel exhaust fluid (DEF). The active ingredient in DEF, urea, converts NOx emissions in the exhaust stream into more environmentally friendly byproducts including nitrogen gas and water vapor.

The final major emissions constituent is particulate matter. Although some particulates are released from the burning of gasoline, particulate matter is significantly more prominent in diesel emissions. Particulate matter from diesel engines is primarily soot; fuel that was not burned completely in the engine. Beginning in 2008, all major manufacturers adopted particulate filters to meet strict Federal mandates on diesel emissions. Diesel particulate filters are 99+% efficient at capturing soot, nearly eliminating particulate emissions in vehicles that employ this technology. Today, you'll find diesel particulate filters on everything from compact cars to large tractor-trailers.

In addition to exhaust aftertreatment systems and clean engine technologies, diesel fuel formulations were also changed in the early 2000's. The current ultra low sulfur diesel fuel available to consumers today is significantly more environmentally friendly than the previous low sulfur diesel classification. Case in point, modern diesels are anything but "dirty".

Diesel engines are fuel guzzlers

False. Diesel engines typically operate at a significantly higher thermal efficiency than comparable gasoline powered internal combustion engines and diesel fuel itself carries a higher energy content per unit of fuel than gasoline. Both of these factors result in favor of diesel with regards to fuel economy. To properly compare fuel efficiency between a diesel and gas powered vehicle, you must take into consideration the engine displacement, output, and vehicle weight. Are you really comparing apples to apples? Diesel powered pickups are generally heavier and have large displacement, high output engines, yet return significantly higher MPGs than trucks with a large displacement gas engine in a similar vehicle class.

Diesel engines are noisy

Generally true, but somewhat misleading. Diesel engines produce a number of distinct sounds, the most prominent of which is the "knock" that occurs when fuel auto-ignites in the combustion chamber. You'll also hear various clanks and ticks as the injectors fire. Diesel engines have become significantly quieter through the years and automakers use lower NVH (noise, vibration, harshness) as a prominent selling point. Diesel engines are typically louder, but significant strides have been made unilaterally to keep engine noise low.

Diesel engines won't start during winter/cold weather

False. Diesel engine manufacturers employ a variety of equipment and techniques to ensure that their engines will start in even frigid cold conditions. These starting aids include glow plugs, intake air heaters, grid heaters, and engine block heaters, all designed to ensure that the engine is capable of starting regardless of ambient temperature. When these systems don't function accordingly or are in need of repair, you'll find that starting a diesel engine may be difficult in the cold. Most, if not all diesel engines are sold with an engine block heater option. These are plugged in for long periods (generally several hours before starting, or even the night before) and raise the engine coolant/engine oil temperature to make starting easier and less cumbersome when winter sets in. Even some gasoline engines are offered with engine block heaters to provide critical engine protection in the extreme cold.

Diesel engines have a higher cost of ownership

Misleading, but with some truth. The maintenance and repair costs are generally higher on diesel engines. Diesel engines often have considerably larger engine oil capacities and multiple fuel filters, which translates into higher maintenance costs. Filters, in general, are more expensive on diesel vehicles because the filtration needs of a diesel fuel system are more stringent. Furthermore, diesel engines are typically a "premium" upgrade on vehicles for which they are offered, so the initial vehicle cost is greater.

Despite these factors, the overall cost of ownership is not necessarily higher. The lifespan of a diesel engine is typically 2 to 5 times that of the average gas engine and a diesel engine is likely to be considerably more fuel efficient. With all things considered, the cost of ownership is going to depend on individual needs and usage. Someone looking to own a vehicle for 5 to 10 years will have an overall lower cost of ownership than an individual whom plans on purchasing a new vehicle every 2 to 3 years.

Diesel engines can run on used vegetable oil

Misleading. Diesel engines can run on waste vegetable oil (WVO), but it must be processed into biodiesel first. The WVO must undergo a process called transesterification, which will cause the glycerin and methyl esters found in vegetable oil to separate; the glycerin is discarded, and the remaining products are used as biodiesel. It's important to make the distinction that raw WVO should not be used as fuel and that it must undergo treatment first.

Diesel engines don't perform well at high altitudes

Misleading and needs additional context. Naturally aspirated engines, in general, do not perform well at high altitudes. Power loss at altitude is not specific to diesel engines; the same laws of physics apply for any internal combustion engine. As altitude increases, the density of air decreases. Since your engine is drawing in "thinner" air, less oxygen is filling the combustion chambers of the engine with each intake stroke. Since turbocharging is a means of forced induction, it compensates for the lower air density.

Most, if not all on-highway engines found in passenger cars and light trucks have been turbocharged since the mid-1990s. Turbocharging was originally introduced as a means of combating power loss at higher altitudes. Turbodiesels generally do not suffer a noticeable loss of power until they reach an altitude of 10,000 feet above sea level, but this figure will vary by manufacturer.

Diesel Engines are slow and sluggish

Misleading and in many cases false. Power-to-weight ratio is the number one determinant of real world vehicle performance. It's difficult to compare a 7000 pound pickup truck to a sports car, so perspective is everything. The high torque output and flat torque curve prominent in diesel engines actually tends to favor acceleration. You'll find that diesel vehicles downshift much less than a comparable gas vehicle to pass, merge onto freeways, or drive up steep grades. Most first time diesel owners are pleasantly surprised with the performance characteristics of their engine, and they reap additional benefits when it comes time to refill the fuel tank.