Contrary to popular opinion, diesel is not new. Diesel engines have a history as old as the automobile itself. However, lately, they have been the focal point of a lot of good press. Most of the aforementioned hype is coming directly from the impressive fuel efficiency. Unlike hybrid, electric and hydrogen power, diesel is a time-tested technology. However, Americans, myself included, love gas-powered vehicles. Although much of this predisposition may come from simply what we are accustomed to, there are many reasons people are reluctant to own a diesel. I will do my best to bring to your attention the advantages and disadvantages of owning a diesel, and give voice to both the diesel and gas crowds.
Let me first begin with a brief explanation of the difference between gasoline and diesel engines. The main difference is obvious: gas engines burn gas, diesel engines use diesel. However, because of the characteristics of both fuels, the two engines use fuel differently.
As many of you are familiar, gas engines found in cars operate on the four strokes: intake, compression, power and exhaust.
- Intake: The piston drops, drawing atomized gas into the combustion chamber.
- Compression: With all valves closed, the piston rises, compressing the gas/air mixture tight inside the combustion chamber.
- Power: A spark plug lights and burns the condensed mixture rapidly, causing the piston to push back down, and creating power.
- Exhaust: The piston rises again, pushing all the waste vapors (exhaust) out of combustion chamber to start the cycle over again.
Diesel works in much the same manner and operates on the four strokes also. However, a diesel engine creates energy in the power stroke without a spark plug. This energy comes from the detonation of the diesel fuel that is caused by such high pressure from the compression stroke. Only air is compressed in a diesel engine and the detonation occurs when the atomized diesel is injected into the compressed and heated air in the combustion chamber. For this reason, diesels have much higher compression ratios. Compression ratios commonly found on gas engines range from 8:1 to 14:1 but those in a diesel engine are much higher from 15:1 to 22:1.
By design, diesels make large amounts of torque and lower amounts of horsepower relative to their size. Additionally, many diesels cannot reach as high in the RPM band. Not coincidentally, diesels are extremely efficient at producing large amounts of low-end torque. For these reasons, turbos are often found on factory diesel engine configurations, to even out power distribution across the power band and make more impressive horsepower specifications.
All the differentiating aspects of driving a diesel can be thought of with two opposing views, those in support and those against moving to a diesel. I will list many of the common pros and cons of a diesel.
Pros | Cons
Fuel Efficiency | Finding Diesel
Longevity | Cold Weather
Towing/Hauling | Model Selection
It Comes with a Turbo | It Comes with a Turbo
Sound | Sound
It is incontestable that diesels make exemplary fuel mileage. Although the new VW Jetta TDI posts only 41 mpg in their official advertisements, I have heard of people putting up numbers as impressive as 55 mpg without modification. The diesel versions of ¾ ton and larger pickups make significantly better mileage than their gasoline powered counterparts. There is no question that if fuel economy is the number one factor in your purchase decision, it makes sense to choose a diesel engine option. However, depending on where you live, diesel can be hard to find and, at times, very expensive. I have a friend with a 2006 TDI Jetta and although he enjoys fuel economy above 40 mpg regularly, he has admitted it is difficult at times to find diesel fuel. In certain parts of the United States, diesel can only found at select truck stops. Also, some truck stops offer diesel but only with monster pumps that don’t accommodate the TDIs fuel door.
One of the defining attributes of the diesel engine is its legendary longevity. Cummins Diesels are designed to last a million miles. Among passenger cars, it is very common that cars rust and break to pieces long before their diesel drive train is ready to throw in the towel. In fact, in parts of the world where diesels are more prevalent, it is frequent for consumers to put a diesel engine in their previously gas-powered car to go another several hundred thousand miles. Despite the diesel’s reputation for durability, and its comparable simplicity, cold weather can become much more of a problem. Due to the consistency of diesel fuel, “gelling” can become an issue. When in extreme cold, the fuel gets thicker and is hard to keep warm enough to flow as needed through the engine. Although advancements in technology have helped in this matter with the advent of glow plugs, diesel fuel treatment and fuel tank return systems, it still can be restrictive. My friend with the 06 Jetta TDI said that when the temperature gets below -10 F, if parked outside, and not applied fuel treatment, it will have trouble starting.
For the SUV and truck applications, diesel can be beneficial those towing and hauling with their vehicles. A steady application, beginning at low RPMs, of torque through the power band, makes towing and hauling smooth and strong. Additionally, tuning companies and aftermarket parts are plentiful for the Cummins, Duramax and Powerstroke diesel engines, making them even more exemplary in a heavy duty towing or hauling situation.
Despite all the apparent advantages of the diesel engine, if this article has spurred you to go find a vehicle you love as soon as possible with a diesel engine; you may find the selection sparse if you live in North America. To my knowledge, most of the sedans and SUVs available in a diesel platform are German, with brands like VW, Mercedes, and BMW producing offerings. Also, Jeep offers a diesel version of its Grand Cherokee. The only other North American diesel products available that I know of are in the ¾ ton and larger pickup truck segment. If you need a domestic sedan or a minivan, you may be hard pressed to find a diesel option for some time.
Now arrive at my favorite part of this discussion. We will discuss the turbo. I have the turbo listed on both the pros and cons sides for several reasons.
First let’s talk about tuning and power. Since diesel fuel is injected, and not drawn in through vacuum, diesels respond extremely well to turbochargers and don’t have the fuel economy losses that you see in some turbocharged gas engines. Since the fuel is injected, whatever quantity of air you can cram into the combustion chamber is acceptable, as long as your fuel delivery system and the engines internals can hold up to the abuse. With a diesel you don’t have to worry about detonation, the bane of boosted gasoline vehicles, because detonation without spark is the goal of a diesel engine. Most diesels come with a turbo or two already and as such, all the pain of plumbing and spatial constraints are already alleviated by the original engineers. Almost unlimited boost potential, with bottom ends that are overbuilt, and plumbing that is already conceived, can be a deadly starting point for serious power.
Unfortunately, there is a downside to a turbo. Although diesel engine technology in its naturally aspirated form is simpler than gas, adding a turbo and all the plumbing means there are more things that could go wrong. In addition to all the additional parts, all these parts need to be lubricated and maintained as well. For a backyard mechanic like me, this could become extremely stressful, despite the potentially balancing joy of a 20:1 compression ratio diesel making 15+ lbs of boost.
The other thing that I listed on both the positive and negative sides of the equation is the sound of a diesel engine. Although I love a throaty rumble of exhaust when I burry my right foot, it must be said that diesel engine noise is an acquired taste. My friend’s TDI Jetta sounds like a low volume school bus at idle. I can’t speak for everyone, but I can say that I personally love the gas inline 6 or v8 growl much more than its diesel counterpart, especially with aftermarket loud exhaust. As much as I’d love the fuel efficiency and torque increase of a diesel in a Super Duty Ford, I just don’t know that I could get used to the sound. For others, it may not be as important and others yet may prefer it.
In summary, I hope you have learned half as much as I have about the advantages and technology of diesel. If for no other reason than to clear the air of intolerance between the pro-gas and pro-diesel crowds, I think we should all respect the advantages and differences. The increase in demand for diesels because of their fuel economy, and power potential has made diesel tuning increasingly popular. Even for those of us that will stick with gas cars for some time, it is good to have an idea of what diesel offers.
I feel that it is only fitting to state where I stand in the gas vs. diesel spectrum as I have taken a positive stance on diesels without being a diesel owner. So here is my personal opinion: I love the gasoline engine. The mainstays on my personal foundation in gasoline engines include the simplicity of natural aspiration, the sound of gas engine exhaust, and the comparatively unlimited model selection. However, I learned a great deal while researching for this article and would love the opportunity to make 40 mpg in an insanely boosted, long-lasting engine. Maybe there is a diesel in my future after all. My hope is that the increasing popularity of diesel VWs, BMWs, Mercedes, and large pickup trucks brings demand for diesel in something like a half ton truck for me or maybe an AWD sedan for my wife. I’ll have to see and how it sounds, how it drives, but I am no longer close-minded to the idea.
