Myths about Diesel and Diesel Engines Debunked

An American Scientific subscriber wrote to the magazine and asked why European cars are not available in the United States. What prompted the question is the fact that many European passengers, “cars are more efficient and better for the environment.” The answer received was short. According to the American Scientific, the reason why U.S. Americans do not drive European cars that are both more fuel efficient and better for the environment is that they do not like diesel. U.S. Americans do not drive diesel-powered vehicles.

“Since the advent of the automobile age in the U.S., gasoline has been king of the road; today upwards of 95 percent of passenger cars and light trucks on American roads are gas-powered. And the federal government has done its part to keep it that way, taxing diesel at a rate about 25 percent higher than gasoline. A recent assessment by the American Petroleum Institute, an oil industry trade group, found that federal taxes accounted for 24.4 cents per gallon of diesel but only 18.4 cents per gallon of gasoline.”

According to the American Scientific, because of higher taxes, the economic advantage of driving diesel-powered vehicles is much less in the United States than other countries around the world. However, ask the average person in the U.S. why they do not drive a diesel-powered passenger vehicle or pickup truck and the answer is not likely to be, “the financial advantage is not great enough for me.” 

Instead, the U.S. Americans perpetuate myths and misconceptions about diesel engines. We typically think of diesel engines as dirty, slow, and loud. When U.S. Americans think of diesel engines, they think of semi and dump trucks and heavy equipment. Most of us don’t think of pickup trucks and passenger cars.

But, maybe it is time we did.

Myths and Misconceptions about Diesel Engines

There are three myths about diesel engines that almost always come to the forefront of the gasoline-vs-diesel debate. The first myth is that diesel engines are dirty. The second is that diesel engines are loud. The third myth is that diesel-powered vehicles are slow. 

Diesel Engines are Not Dirty Relative to Their Gasoline Counterparts

The claim that diesel engines are dirty is not — and never was — true, at least not in relation to gasoline engines. It is true that the emissions from diesel engines are different than the emissions from gasoline engines, but the word “dirty” is a poor choice of words. Gasoline engines actually produce far more of some types of greenhouse gases that are very well known to us.

But, the misconception that diesel is dirty is perpetuated and preserved to the extreme.

For example, in 2017 Sadiq Khan — the Mayor of London — said, “The science now tells us that diesel vehicles cause more than four times the pollution than petrol cars.” The fact is however, the science tells us quite the opposite. Diesel engines are not only cleaner than gasoline engines today, diesel-powered vehicles have always been cleaner than gasoline-powered vehicles of comparable size, at least with respect to the greenhouse gas that has us most concerned, carbon dioxide. 

Diesel Engines Produce 40% Less CO2

Diesel engines produce more emissions than gasoline with respect to a number of different types. But, carbon dioxide is not one of them and carbon dioxide is — right now — our biggest concern. Carbon dioxide is the emission having the greatest impact on the environment. The reason most people are not aware that their gasoline vehicle produces far more carbon dioxide than its diesel counterpart is because most CO₂ statistics are per-unit-of-volume stats. 

Per gallon, diesel engines produce slightly more CO₂ than gasoline engines. But, thinking about CO₂ emissions on a per-unit-of-volume is grossly misleading. Diesel engines are far more fuel efficient than their gasoline counterparts. Instead of asking how much CO₂ diesel produces per gallon, the question that should be asked is how much CO₂ diesel engines produce per mile. 

Paul Nieuwenhuis, Senior Lecturer and Co-Director of Electric Vehicle Centre of Excellence (EVCE), Cardiff University explains, “Diesel engines are “lean-burn”, meaning they use less fuel and more air to get the same performance as a petrol engine. So, while diesel fuel contains slightly more carbon (2.68kg CO₂/liter) than petrol (2.31kg CO₂/liter), overall CO₂ emissions of a diesel car tend to be lower. In use, on average, this equates to around 200g CO₂/km for petrol and 120g CO₂/km for diesel.”

Gasoline engines, that means, are 40% dirtier with respect to CO2 emissions than diesel. 

But, diesel engines are not the only cleaner with respect to CO2 emissions than gasoline engines, diesel engines also produce fewer hydrocarbon emissions. 

Diesel Engines Produce Fewer Hydrocarbon Emissions

Hydrocarbons are the molecules and molecular chains in fossil fuel that ignite/burn/combust. Hydrocarbons are the reason that fossil fuels have value. However, hydrocarbons are also a form of emissions. When hydrocarbons enter a combustion engine, but are not burned or are only partially burned, hydrocarbons are blown out the exhaust as dangerous, polluting, and toxic emissions. 

By design, gasoline engines produce far more hydrocarbon emissions than diesel engines. It is, in fact, necessary for gasoline engines to produce hydrocarbon emissions. Gasoline engines must run dirty. The reason being, gasoline engines cannot run at optimal air-to-gasoline ratios. If a gasoline engine operates with an air-to-gasoline ratio that will produce a 100 percent clean burn of the hydrocarbons, a gasoline-powered engine will overheat, burn up, and destroy itself.

Stoichiometric Mixture: Diesel vs Gasoline Engines

A stoichiometric mixture is the ideal mixture of fuel and air. In order for hydrocarbons to burn — meaning in order for any fossil fuel to burn — it must be oxygenated. Without oxygen, hydrocarbons — again, fossil fuels — will not combust. If there is some oxygen in an engine’s fuel-to-air mixture, but not enough to oxygenate all the hydrocarbons in the fuel, the resulting emissions will contain unburned and partially burned hydrocarbons. 

If there is more oxygen in a fuel-to-air mixture than is necessary to oxygenate all the hydrocarbons in the fuel — if there is extra oxygen, — the lack of fuel in the mixture will cause a gasoline engine to run poorly because there are not enough hydrocarbons in the mixture to fire the pistons. However, the biggest issue with gasoline engines with regard to emissions is the fact that gasoline engines cannot run at an ideal fuel-to-air mixture ratio.

The perfect mix of fuel-to-air is called a stoichiometric mixture. A stoichiometric mixture is an exact number of oxygen molecules in a fuel-to-air mixture to oxygenate all the hydrocarbons in a fuel without any extra oxygen left over. Though a stoichiometric mixture is ideal with respect to fuel efficiency and emissions, gasoline engines cannot operate on a stoichiometric ratio mixture. 

The stoichiometric mixture is not only the perfect oxygen to hydrocarbon ratio; it is also the mixture that burns the hottest. Gasoline engines naturally run hotter than diesel engines — even though diesel burns much hotter than gasoline because it has more energy. But, if a gasoline engine — a spark-fired engine — operates on a stoichiometric mixture, it will burn up. The heat generated by a stoichiometric mixture will destroy a spark-fired gasoline engine. 

So, gasoline engines must run rich. All gasoline engines are designed to burn an air-to-fuel mixture that has less oxygen than is required to oxygenate all the hydrocarbons in the air-to-fuel mixture. That means gasoline engines are designed to create emissions that contain unburned and partially burned hydrocarbons. Not only does intentionally blowing unburned “fuel” out the exhaust cause environmental issues, it is a tremendous waste of fuel — a tremendous waste of energy. 

Diesel engines — on the other hand — can operate on a stoichiometric mixture. Diesel engines can run cleanly without the heat from the engine destroying the motor. In fact, not only is it not necessary to run diesel engines rich, diesel engines can run lean. 

Diesel engines — compression engines — can run super lean. The reason being, the amount of air in a diesel engine is constant. The amount of fuel injected into the cylinders changes depending on the amount required. In a gasoline engine, air and fuel are mixed before being injected into the cylinders. So, the ratio of fuel-to-air remains the same. In a diesel engine, the fuel-to-air mixture ratio changes constantly. 

Often, the fuel-to-air mixture in a diesel engine is lean which means a complete fuel burn. A complete fuel burn means zero hydrocarbon emissions and greater fuel efficiency.

But, the notion that diesel engines are dirty is not the only myth. U.S. Americans also believe diesel engines are slow.

Diesel-Powered Vehicles are Slower than Gasoline, but No Longer by Much

Diesel engines were once much slower than gasoline engines. But, with the advent of turbo diesel engines, that is no longer the case. Diesel engines are now comparable to gasoline engines with respect to acceleration rates, though gasoline engines still have higher top-end speeds. The reason being, gasoline engines have more horsepower than their diesel counterparts. 

There is a difference between torque — of which diesel engines have considerably more at low rpm than gasoline engines — and horsepower. Torque is the amount of force that an engine exerts to turns an axel. If a gasoline engine and a diesel engine operating at the same rpm, the diesel engine will produce considerably more torque than a gasoline engine because diesel engines have a longer piston stroke, i.e. a greater compression ratio. The longer the piston stroke, the greater the torque. 

Horsepower is the sum of torque and the rpm of an engine. Diesel engines have very low rpm operating spectrums. Gasoline engines — because they have shorter piston strokes — have higher top-end rpm than diesel engines. The power band of a diesel engine — the rpm rate at which a diesel engine generates the most power — is at around 4,000 rpm. The power band of a gasoline engine is between 5,000 and 6,500 rpm. 

The greater the rpm, the greater the horsepower. The more horsepower a vehicle has, the higher it’s top-end speed. But, adding a turbo to a diesel engine increases the rpm of a diesel engine. So, today’s diesel engines are comparable to gasoline engines. 

The 2017 Porsche Panamera 4S Diesel, “is the quickest diesel you can currently buy, hitting 0-62mph in just 4.5 seconds and topping out at 177 mph thanks to its 4.0-litre V8. Plus, it returns an impressive 42.1mpg on average and features a spacious, plush cabin and four-door practicality.” The 2017 Porsche Panamera 4S Gasoline does 0-60 in 4.2 seconds, hardly significantly faster.

Diesel Engines are No Longer Loud in Relation to Their Gasoline Counterparts

Maybe the biggest knock on diesel passenger vehicles is that they are loud. But, today’s diesel engines are difficult to tell apart from gasoline engines when driving. The 2017 BMW 5-Series, for example, is exceptionally quiet. “The only time you can tell it’s a diesel is when you start the engine or sit there idling at the traffic lights. At idle, there is a subtle, faint diesel chatter that gives it away with almost no vibration. Push the tach beyond 2000 rpm, however, and the tell-tale diesel sound disappears completely with no vibration whatsoever. And when accelerating hard, the engine feels and sounds like a gasoline engine.”

While the diesel of the past was loud and slow, that is no longer the case. And, diesel engines — hence diesel — has not ever been a dirty fuel.


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