A diesel’s strengths, weaknesses and misconceptions.
Mike Sefton – CleanMPG
– Aug 13, 2008
My 2001 Volkswagen Golf TDI.
A short introduction to diesel engined automobiles. I drive a 2001 Volkswagen Golf TDI (Turbocharged Direct Injection diesel) and frequently find myself explaining to people what it means for an engine to run on diesel. We will cover some of the most common points that I discuss with others as well as some points that I have learned through ownership.
Diesel History in brief
The diesel engine gets its name from Rudolf Diesel, who filed for a patent on his engine design in 1894. Seeking to invent a more efficient engine, Diesel came up with one that relied on compression to ignite the fuel. Diesel’s early engines ran on a biofuel made from peanut oil. A frequently repeated quote from Diesel is “The use of vegetable oils for engine fuels may seem insignificant today, but such oils may become in the course of time as important as the petroleum and coal tar products of the present time.“
What makes a diesel engine a diesel?
The main difference between diesel and gasoline engines is that a gasoline engine uses spark plugs to ignite the air / fuel mixture in the combustion chamber, while a diesel engine does not. Instead, a diesel engine compresses air in the combustion chamber, which causes the air to become extremely hot. At just the right point, fuel is injected into the combustion chamber where it hits the hot air, causing the fuel to burn. This is one reason why diesel engines make more noise than gasoline engines. In the gasoline engine, a flame propagates out from the spark plug through the combustion chamber. In a diesel engine, everything combusts more or less at the same time, which leads to a much more sudden rise in temperature and pressure and more noise.
Cutaway of a direct injection diesel engine. Shows fuel injector, glow plug, piston, and valve.
There are a few notions that many people have about diesel engines that may have been true at one point, but are not any more. The following four are the ones I hear most frequently. Emissions also comes up a lot in discussions but has been left out of the misconceptions because it has its own section later on.
Diesels are not fuel efficient
I don’t know where this one comes from. It is simply not true. A diesel engine gets approximately 30% better fuel economy than a gasoline engine with the same horsepower.
Diesels are noisy
Now this one is partially true, especially for older engines. However, modern diesel engines are much quieter than their predecessors. From the outside of a vehicle, even the best new diesels make noticeable noise, but not enough to be obtrusive or overpower a conversation. Inside the vehicle at idle, a new diesel is still noticeable to occupants if they listen closely. Once the vehicle is underway, road and wind noise will overpower most differences in engine noise. Inside the vehicle at highway cruising speed, a diesel powered vehicle is often quieter than the gasoline version because the engine is running at a lower speed. Again, this is rarely noticeable to anything but a sound pressure level meter and radio or ventilation fans cover up much of the difference.
My car uses an engine and fuel injection system that first came out in the early to mid 1990’s. As a result, the engine is far louder than the new diesels from 2007 and later. Even so, passengers do not notice my car is diesel powered until I have to explain to them why my gas gauge doesn’t seem to move or why my tachometer only goes up to 5000 rpm.
Diesels are hard to start in cold weather
This is also partially true for older engines. Diesel engines have a “wait to start” or “glow plug” light that indicates that glow plugs in the engine are preheating and that the driver should not attempt to start the engine until the light goes out. Older engines such as mine do not need to wait above 40F, but may need 5-10 seconds at 32F and 10-20 seconds at -10F. Newer engines are much faster; waiting times of about 3 seconds at -20F and no waiting above 32F are common. The owner can spend the 3 second waiting period buckling a seatbelt and the car will then be ready to go. Having the correct battery is important for starting a diesel reliably. If a battery needs replacing, ensure to get one with enough reserve capacity. Many people overlook that and only consider cranking amps, which will seem to work until a cold day makes the glow plugs run for an extended period.
Personally, my car has never failed to start when cold and not plugged in, even on those bad -30 Winnipeg winter days that we see a few times each year. When below 40F, I spend the 5-15 seconds of preheating buckling my seatbelt and checking my mirrors. In winter, I try to plug my block heater in a couple hours before needing my car. Running the block heater on cold days eliminates the preheating time and makes the car start like on a summer day.
So what does this preheating cycle actually do? Remember from the introduction that diesel engines compress the air inside the combustion chamber to heat it up and burn the fuel. On cold days, the air will not get heated to as high of a temperature and the cold metal from the engine will suck much more heat out of the air before the fuel is injected. If the air in the combustion chamber is even one degree below the ignition temperature of the fuel, the fuel will not ignite. This is where glow plugs come in. They get extremely hot (over 1800F in only 5 seconds!) to create a small hot spot in the combustion chamber to help ignite the fuel when the engine is cold.
Diesels are slow
A glow plug glowing white hot
Like any other engine, some diesels are built for fuel economy and some are built for speed. A good example of this can be found in the European BMW 3-series. The base diesel engine in the 318d makes 143hp, goes 0-60mph in 9.3s, and is rated at 50mpg US. The top diesel engine in the 335d makes 286hp, goes 0-60mph in 6.1s, and is rated at 35mpg US. In the past, North Americans have usually seen the engines which were intended for good fuel economy, such as the 90-100hp VW 1.9 TDI. Diesel engines coming to North America in the next few years will be much more powerful than older engines, but may sacrifice some fuel economy.
A diesel engine often feels faster than it is because of how it develops its power. Much of its power is available at very low engine speeds, where most driving takes place. Therefore, the engine feels very responsive in daily driving situations. On paper, my 90hp TDI looks underpowered but it never feels that way when driving. Some people who have driven it have actually said that it “hauls butt", which I found amusing.
As much as I like my TDI there are still downsides to having a diesel powered vehicle. Most of them relate to fueling:
- Not all fuel stations sell diesel. To find fuel reliably in unfamiliar areas, it is best to fill at truck stops.
- Diesel fuel can gel if it gets too cold. This is generally not a problem for most people as fuel stations will winterize fuel enough for the local climate as weather gets colder. In a situation like driving from Texas to Minnesota in January, it is best to fill with local fuel as the vehicle gets further north. Additives can also help to prevent fuel from gelling.
- Because diesel engines are more efficient than gasoline engines, they make less waste heat. This means that in winter, it will take longer for the engine to warm up which means longer for the cabin heater to start making significant heat. This is easily remedied with a block heater, which is something that every aspiring hypermiler in cooler climates should purchase anyway.
- Because a diesel pump nozzle is larger than a gas nozzle, it is possible to fill a diesel fuel tank with gasoline, so always watch what goes into the tank. Running a diesel engine on gasoline can cause serious damage to the fuel injection system. An incorrect fill actually happened to me in October. I told the attendant to fill the car with diesel and went into the station to take a look at the newspaper. A few minutes later, he came in and asked who had the diesel Golf, which immediately got me worried. He told me he had filled the tank with gasoline by accident. After the attendant unsuccessfully tried to pump the tank out for about an hour, the manager came by and lent me his van for the weekend. My car got towed to a mechanic who emptied the tank and then refilled with diesel and fuel conditioner. After that incident, I always fill my tank myself. Some states such as New Jersey and Oregon do not allow motorists to pump their own gasoline because it is flammable. Diesel is a combustible liquid but not flammable, so a motorist is permitted to pump diesel in those states.
- Not all mechanics are qualified to work on a diesel car. It is best to take the car to a shop that knows the car well. Some mechanics claim to know diesels when most of their business is Mercedes diesels from the 1980's. Experience with those older engines is often not enough to work on a new clean diesel. Some quick oil change shops unfamiliar with the car may not even realize the car has a diesel engine and they could put in the incorrect oil.
This is another important one for me. Frequently, when people think of a vehicle with a diesel engine, they picture a stinky old bus or truck that can envelop small cars with its soot. Fortunately, modern diesels in both cars and trucks have cleaned up tremendously since those old engines. The following will focus only on the clean diesel powered cars, meeting the EPA’s Tier 2 Bin 5 standard or better.
Weak points of a diesel engine’s emissions have always been particulate matter and nitrogen oxides. Particulate matter is made up of fine particles and makes up much of the dark visible exhaust coming out of the tailpipe. It is linked to respiratory problems like asthma and lung cancer. Nitrogen oxides are a component that contributes to smog. For both forms of emissions, clean diesels are on par with a typical gasoline car such as a Camry or Civic, but still emit more than what a fuel efficient hybrid such as a Prius or Civic Hybrid will emit. An area where diesels tend to do well is evaporative hydrocarbon emissions, which contribute to smog. This is because gasoline evaporates much more readily than diesel fuel, so the handling of gasoline produces more evaporative emissions. When considering greenhouse gases such as carbon dioxide and methane, a diesel engine will have an advantage over a conventional gasoline engine. This is because the diesel engine will burn less fuel.
Diesel Cost Analysis
Possibly one of the most important areas for many people is how much more money they will have to spend to get a diesel vehicle. This section will compare gasoline and diesel versions of a vehicle to show that getting a diesel engine does pay off financially.
I have found that maintenance costs for my car are similar to what a gasoline version would require, so they will be assumed to be equal for this comparison. People frequently ask about oil change intervals and costs. Diesel engines will need synthetic oil which is more expensive than conventional oil, but it requires changing less often which makes up the cost difference.
Fuel prices are the national average pulled from AAA’s website on June 8, 2009. Three clean diesels will be looked at: The 2009 Volkswagen Jetta TD, BMW 335d, and 2009 Mercedes E320 Bluetec. More comparisons may be added as more models become available.
2009 VW Jetta TDI
Engine upgrade cost: $2250
Federal tax credit: $1300
Net upgrade cost: $950
2009 BMW 335d
Engine upgrade cost: $2275
Federal tax credit: $900
Net upgrade cost: $1375
2009 Mercedes E320 Bluetec
Engine upgrade cost: $1000
No federal tax credit
Regular Unleaded: $2.613/gal
Cost comparison: Gas vs. Diesel
|Make||Model||Engine Size(Liter)||Engine type||Transmission||Upgrade Cost ||EPA combined fuel economy (*MPG)||Fuel cost (cents per mile)||Payback time (miles)|
|| || || || || || || || |
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* 08 EPA Fuel Economy estimates.
So even in a purely financial sense, buying a diesel powered vehicle can be worth it. It is important to note that this comparison does not consider resale values, which will also favor the diesel powered vehicles. Currently, there is a high demand for diesel vehicles so the engine upgrade cost appears to be appreciating. For example, the engine in my car was a $1100 upgrade but it is worth about $2000 more than the gasoline version at the moment. Even without the current high demand, used diesels have traditionally sold for more than used gasoline cars by an amount similar to the original difference in purchase prices.
Biodiesel is a biofuel made from transesterified vegetable oils. Commonly used feedstocks in producing biodiesel are soy, canola (rapeseed), and waste vegetable oil. It is important to note that biodiesel is not the same as waste vegetable oil or straight vegetable oil, which will be discussed in the next section. Diesel cars made after the mid 1990's generally do not need any modification to run on biodiesel. Diesels from the early 1990’s or earlier may require rubber fuel lines to be replaced, as biodiesel can cause them to degrade over time. Biodiesel contains approximately 7% less energy than petroleum diesel. In blends with low amounts of biodiesel such as B2 (2% biodiesel) or B5, many people have seen an increase in fuel economy. This is because biodiesel has better lubricity than petroleum diesel, which offsets the lower energy content. Blends of B20 and above usually begin to show the effects of decreased energy in the fuel and fuel economy will decrease slightly. Biodiesel has a better energy balance than ethanol, which means its ratio of energy produced to energy consumed is higher.
Benefits of running biodiesel include lower greenhouse gas emissions and particulate matter emissions. The increased lubricity helps to extend the life of fuel injection equipment. Drawbacks of biodiesel are that it increases nitrogen oxide emissions and that it is more susceptible to gelling when the weather gets cold. B5 blends have been used in Manitoba winters down to temperatures of -40F, but care should be taken when experimenting with biodiesel in cold weather for the first time. Biodiesel is also an excellent solvent and can dissolve residue left behind by petroleum diesel. The downside of this is that the dissolved residue may build up in the fuel filter, clogging it prematurely. Once all residue has been dissolved, fuel filter replacements should be no more frequent than what a car running on petroleum diesel will need.
Currently, clean diesel cars from Mercedes and Volkswagen sold in North America only allow blends up to B5 under warranty. The reason is because manufacturers are concerned that high levels of biodiesel may dilute the engine oil when used in engines with certain emissions control systems. In those systems, fuel is injected after combustion, with the intention of having the fuel vaporize but not burn, then exit through the exhaust valves and travel down the exhaust stream to the particulate filter where it is used to incinerate the collected soot. Since biodiesel has a slightly higher boiling point than petroleum diesel, it will not vaporize as readily and may make its way into the crankcase and dilute the motor oil. Therefore, B5 was determined to be the limit for acceptable oil dilution levels in many new cars. Any biodiesel must be made to ASTM D6751 standards in order to be acceptable.
Straight / Waste Vegetable Oil
Straight vegetable oil (SVO) and waste vegetable oil (WVO) are vegetable oils that have not been transesterified. Typically, the fuel is filtered before being put into a dedicated vegetable oil tank in the car. The diesel engine is warmed up on diesel, and heat from the engine is used to warm the vegetable oil to make it less viscous. Once the vegetable oil is warm, the engine can be switched over to run on vegetable oil. Before the engine is shut off, the vegetable oil must be purged by running the engine on diesel fuel.
For modern direct injection diesel engines running on vegetable oil, durability problems seem to arise more frequently than many are comfortable with. The following is a description of what can happen with a direct injection engine running on waste vegetable oil: Direct injection diesel engines operate at very high fuel injection pressures (10000-30000 psi) and fuel is squirted through extremely small holes in the fuel injector nozzle. The extreme pressure and heat of fuel injection and combustion can cause the vegetable oil to break down, which in turn can lead to carbon deposits on the fuel injector nozzles. The carbon deposits disrupt the fuel injection so that instead of a finely atomized spray, the nozzle lets out a jet of poorly atomized fuel. Some of the spray may come in contact with the cylinder wall, where it remains until it is scraped off by the piston ring. Over time, heat and pressure from in the combustion chamber cause this vegetable oil to solidify near the piston ring, which prevents the piston ring from forming an optimal seal. As things progress, compression may be reduced which will lead to power loss and poor cold weather starting. Eventually, vegetable oil can begin to get by all rings on the piston and into the oil pan. At some point, heat and pressure from the engine may cause the vegetable oil polymerize in the oil pan, creating a thick gooey mess which will reduce lubrication and can become very expensive.
Note that the above is a description of what can
happen to a direct injection engine. There are examples of direct injection engines running vegetable oil will no ill effects, but there are also many examples of engines which needed expensive repairs after less than 50000 miles on a vegetable oil setup that was thought to be very well done. The failure rate is far too high in my mind for it to be worth the risk. If you must run vegetable oil in your direct injection engine, please read up very carefully on how to do it properly. Make sure to do thorough purges before shutting down and watch your compression and oil carefully. Hopefully you will be among those who don’t see any significant problems.
With that said, waste vegetable oil systems have met with much more success on indirect injection engines. Those engines have a much lower fuel injection pressure and the fuel is injected in such a way that it does not spray into the main combustion chamber, so the potential for ring damage is much lower. Mercedes diesels from the 1980’s are popular cars for vegetable oil conversion, and any non-TDI Volkswagen does not have direct injection and is therefore better suited for conversion than a TDI.
VW 1.9 TDI engines which experienced problems when running a WVO system
Carbon buildup on intake valves------------------------------------------
Polymerized oil in the oil pan
Deposits on fuel injector nozzle-----------------------------------------------
Fuel injector nozzle after cleaning
Manufacturers with plans to sell clean diesel cars in North America include Audi, BMW, Mercedes-Benz, and Volkswagen. Some models are available now and others will appear in the next 1-2 years.
The preceding has been an introduction to diesel engines. I hope you have found it valuable and learned something from reading it. If you have any questions or comments, please do not hesitate to ask. I may not know the answer, but I may know where to find it for you.