Diesels are named after their creator, Rudolf Diesel
. In marine
applications, diesels are the engines of choice for everything except warship
s. The same for heavy trucks, construction equipment
and just about everything else that requires a lot of power. Diesel engines power many automobile
s in Europe
. In America, only Volkswagen
currently offers a diesel engine for its cars. Diesels are an expensive
, but worthwhile option
on 3/4 and full ton rated pick-up truck
s. But that's it. Why?
Diesel engines are a form of conventional, four stroke internal combustion engines. The biggest differences between them and a gasoline engine is in ignition, compression ratio and fuel.
When the intake valve closes and the piston begins moving upward, the fuel and air in all internal combustion engines compresses by Boyle's Law. Gay Lussac predicted compression in turn increases temperature. Since we are compressing fuel and air, once the mixture gets hot enough it will explode.
In a gas engine, fuel/air ignition is controlled through spark timing. The spark is timed so that the fuel/air mixture explodes right when the piston reaches the peak of the compression cycle, or top dead center. If the explosion occurs at any other time power will be lost. If the spark occurs too early, the explosion will tend to shove the piston down when its natural rotation hasn't finished taking it up. That is very hard on the crankshaft, bearings, pistons and rods. An early explosion is called pre-ignition, or pinging.
Several factors can cause pre-ignition. The first is the resistance of the fuel to ignition. Gasoline is more volatile than diesel, thus explodes at lower temperatures. Increasing the compression ratio increases efficiency but also turns up the heat inside the combustion chamber because temperature increases as a gas is compressed. That limits the compression ratio of gasoline fired street cars to around 10:1. The explosion temperature of gas and air is also a property of the fuel/air mixture-- within normal limits increasing the fraction of air to fuel increases temperature, and with it the likelihood of pre-ignition. Air intake in a gas engine is choked off at part throttle to keep the air/fuel mixture correct. The restricted inlet increases drag for the air that is taken in. Engineers call that drag pumping loss. In a gas engine, pumping loss must be accepted. Too lean operation can lead to pre-ignition. For a diesel mixture is irrelevant, so there is no 'part throttle' air restriction, and thus no pumping loss.
Gasoline engines excel at producing horsepower which is defined as about 550 ft/lbs per second, or about 3/4 of a kilowatt. Horsepower is work done over time, and a small amount of energy can become significant through the miracle of gearing. They're better at going fast. Gas engines produce less torque but produce their torque across a relatively wide power band. The spark ignition is good for cold starts. These advantages and history explain why most automobile engines are fueled by gasoline,
Diesel engines operate differently. Diesels control power by metering fuel, rather than air. Air to fuel ratio is for practical purposes, irrelevant. If it takes six molecules of diesel to idle the engine, the fuel delivery system is free to deliver precisely six molecules of diesel, with no waste. No vacuum is thrown away, for the engine will take in all the air it can get, and burn only what is required to ignite six molecules of #2 diesel. There is no need for an ignition system. Compression ignites the fuel. Diesels run a much higher compression ratio than gas engines, over 20-1 and the greater the compression ratio the more power can be extracted from the fuel. They very high temperature process is inherently very clean in terms of hydrocarbon and other forms of air pollution, with the exception of nitrogen and particulate matter. Diesels are very strong torque producers, which explains their popularity in trucks and heavy equipment.
Diesel engines produce similar fuel mileage to gas-electric hybrid vehicles, without the complexity, the need to carry batteries or other expensive items. By rights Americans ought to love them. But we don't. Why?
The American experience with automotive diesels began in the 1970s, as twin oil shocks following the Yom Kippur War and the Iranian revolution sent gas prices soaring during a period of historical recession.
The 1970's represented the nadir of the American automobile industry. Automotive engineers faced three competing challenges. First American car companies made a lot of money off of big, powerful cars. These were seen by the industry, and most Americans as the cars to have. But Americans had become aware that air pollution was producing significant problems. SMOG blanketed many large American cities, most notably Los Angeles. America's politics were fairly liberal at the time, so it was decided to do something about this problem by limiting automotive pollutants, seen as the biggest contributor at the time. Lead poisoning was also seen as a problem, so lead was also banned. However, tetraethyl lead was used both as a lubricant and a combustion inhibitor. Banning it forced gas motors to cut their compression ratios at a time when high compression would have been useful.
Automotive designers labored under technical handicaps that have since been overcome. Most air/fuel mixing and metering was performed by carburetors, traditional vacuum driven devices that are relatively crude. Fuel injection had been around for years, but it was very expensive, more complex and really not much better than carburetors given that all fuel metering at the time was mechanical. Catalytic Converters had not been developed. Microprocessors were something you read about in Popular Science
By 1973 most automobile engines had become a spaghetti-bowl of vacuum lines and pumps as engineers tried to gimmick their way to pollution control. They knew what they needed, catalysts, electronic fuel injection and electronic spark control, but they could not deliver those things at a reasonable cost or reliability before the mid-to-late 1980's.
Then Egypt and Syria invaded Israel during the Jewish festival of Yom Kippur. They achieved tactical surprise and did very well before the tide was turned. Fearing that Israel might fall as Vietnam was starting to wind down President Richard M. Nixon ordered Israel re-supplied, and moved America to Defcon 2 in order to deter a supposed Soviet intervention. Saudi Arabia's King Faisal responded to the re-supply by declaring jihad, which for Saudi Arabia meant they stopped shipping oil to an increasingly dependent West.
Gas prices doubled overnight. Lines formed around gas stations. The economy convulsed. Overnight Detroit engineers who had been stretched to the limit trying to control pollution found themselves forced to crank up the fuel mileage as well. At the same time they had to motivate cars only slghtly smaller than Moby Dick.
They couldn't do it. The logical solution was to build a small car, but GM's experience with the Corvair had made everyone wary of small cars. They had oodles of tooling for big cars and showrooms full of them. They had no experience building small, high revving engines in an era when a 3.5 or 4 liter six was considered a small motor. Salesmen didn't know how to sell small cars. Europe and Japan, long used to building cars for a high fuel prices and under heavy regulation were well used to building such cars. The Japanese manufacturers in particular made inroads into the American market, inroads they have expanded.
Desperation forced American engineers to try some very radical ideas. Cadillac tried the very complex-- and expensive--solution of shutting down cylinders to turn a V-8 into a six or four cylinder engine with a lot of reciprocating mass. They tried sawing the trunk off of big cars, wihch saved about a hundred pounds at the cost of a useful trunk.
GM turned to the diesel. At the time fuel metering systems for diesels weren't a bit more advanced than for gas cars. The best diesels of the day produced great fuel mileage, but power arrived only within a very narrow power band. Horsepower was less than half that of a gas car. The fuel distributors of the day gave maintenance headaches that were easily handled by a trucking company but completely new to most auto mechanic, including at the dealers. The auto diesels endued up being debugged after the sale. And then there came winter.
To start a cold diesel in 1980 you had to hit the glow plug switch, then wait a couple minutes for the cylinders to get hot enough to ignite the fuel. In winter, it seemed like eternity. Sometimes you had to hit the glow plugs more than once before Mr. Motor decided to wake up and come play. When it's five below, that isn't fun. On top of all that some automotive diesels were simply gas engines with the distributor removed and a diesel fuel distributor cobbled on. They did not prove very reliable. They're noisy. Service stations that sold diesel were scarce.
The impression many Americans have of automotive diesels was formed in those dark days. They see diesels as cars that don't want to start and get you from zero to sixty in a about a day and a half. Diesels made their appearance in the market and disappeared very quickly.
Except for Volkswagen and in trucks.
Around 1993 things changed. Catalysts had arrived. Fast glow plugs. Electronic injection arrived. Injectors arrived that could do multiple squirts at high enough pressures to be inserted late in the compression cycle. Turbocharging became reliable.
The automotive and light truck diesel underwent a transformation. Gas mileage improved, and while peak horsepower remains significantly lower in comparison to gas motors, they power band widened and horsepower more than doubled across the power band. Torque increased as well. Now about 100 foot pounds per liter is normal.
In 2002 I test drove, and almost bought, a Volkswagen Golf with the TDI diesel. Nothing could have been farther from my 1982 experience. The car wasn't a hot rod, but it had plenty of power with the air conditioner on. You could pass when needed. It drove just like a regular car, only it got 50 miles per gallon on the highway. A nice, comfortable car for just under $20K. My friends' light truck experience matched the TDI. Today a diesel 3/4 or one ton truck can approach 20 MPG, and still have enough grunt to tow a house. In trucks, diesels have become the stud engine.
But not in cars. Americans remain skeptical of diesels, instead preferring hybrids. And the future does not look bright. Particulate emissions remain a particular problem. But the real problem here is American diesel fuel, which is extraordinarily high in sulfur. American diesel has between a sulfur content of between 140 and 340 parts per million (ppm). European diesel can be as low as 10 ppm, and American diesel won't get close before 2006 when the standard will be 15 ppm. If that date is made, and it may be delayed for political reasons. Clean diesel fuel is essential, but won't be enough to bring it into particulate and nitrogen compliance without other techs, including very radical catalysts.
Modern diesels offer relative simplicity, great torque and outstanding fuel economy with the driveabilty required to make driving fun. In Europe they are common. In America, they ought to be common. Instead the automotive diesel is an endangered species.
Skinwalker has reminded me that Mercedes-Benz sold a number of diesels in the US from 1977 to 1985 without the glow plug, reliability or durability issues observed in more pedestrian cars. Fair enough, though Benz's are also premium vehicles, not built to mainstream cost constraints. Mercedes makes diesels for the European market, hopefully some will find there way here, as they did in the 1960's.