Diesel wastes less of its fuel than almost any practical engine and still gave carmakers their biggest scandal.
Diesel engines are the most thermally efficient practical combustion engine, while petrol engines manage about 20-30%. The cost is emissions: untreated diesels emit 20 times more NOx, and they need particulate filters that struggle on short city trips. Dieselgate showed how far one maker went to hide it.
A diesel engine wastes less of its fuel than any other practical engine, yet an untreated one emits 20 times more nitrogen oxide than a petrol engine. That single tension explains filters, scandals, city charges and a lot of showroom confusion.
How does a diesel engine differ from a petrol engine?
A petrol engine lights a mixture of air and fuel with a spark. A diesel engine compresses air alone, or air mixed with recirculated exhaust gas, until it is hot enough that fuel sprayed into the cylinder ignites by itself. That is why a diesel is called a compression-ignition engine, and why it normally runs a higher compression ratio than a petrol unit.
Rudolf Diesel got the idea in 1878, when a lecturer showed his class a fire piston, a traditional fire starter that squeezes air so fast it gets hot enough to light tinder. The same trick, pumped up to engine scale, is what happens in every diesel cylinder. The same lecturer pointed out that steam engines convert just 6-10 per cent of their heat into work, and Diesel set out to build something far better.
There is a second difference in how the two are driven. A petrol engine controls its output by throttling the air it breathes. A diesel leaves the intake open and changes the amount of fuel injected, so it nearly always burns with a surplus of air. That lean burn is one reason diesels are efficient, and it is also why they behave so differently in the exhaust.
Diesel is the most efficient practical engine ever built, and also the one most expensive to clean up.
Is petrol or diesel more efficient?
Diesel wins on engine efficiency. The diesel has the highest thermal efficiency of any practical internal or external combustion engine, thanks to its very high expansion ratio and lean burn, and large low-speed marine versions reach effective efficiencies of up to 55 per cent. Petrol engines typically manage about 20-30 per cent, roughly half that of some diesels.
The early history shows how hard that was to achieve. Diesel's engine reached an effective efficiency of 16.6 per cent in June 1895. By the main test on 17 February 1897 the figure had climbed to 26.2 per cent, with a rating of 13.1 kW. The first ignition, in August 1893, had used petrol and wrecked the instrument measuring it. A year later Diesel's engine ran for 88 revolutions, a single minute, and the news was enough to send his builder's shares up by 30 per cent. By 1898 he was a millionaire.
In everyday cars the efficiency shows up as lower fuel and CO2 numbers. Diesel engines generally have better fuel efficiency and emit less carbon dioxide than petrol engines. Volkswagen's diesel fleet helped it register a corporate average fuel economy of 34 mpg-US (6.9 L/100 km; 41 mpg-imp) in 2014, though that figure was later bound up in the scandal. Since the 1970s energy crisis, most major automakers have offered diesel models at some point, even in very small cars, and diesels made up around half of new registrations in the European Union at the time of Konrad Reif's 2012 report. In the United States the picture is different: emissions are harder to control, so diesel use is largely relegated to larger on-road and off-road vehicles.
How did diesel become a car engine?
Slowly. Petrol came first, with Nicolaus August Otto and Eugen Langen building the first practical petrol engine in Germany in 1876. Diesel published his rational heat motor essay in 1893 and was heavily criticised for it. His original constant-temperature cycle could never have done useful work, and by June 1893 he had adopted a constant-pressure cycle. Critics went further and called the invention a fraud. Emil Capitaine lost a patent suit against him, and the historian Friedrich Sass calls the engine Diesel's very own work.
The diesel started as a more efficient replacement for stationary steam engines. Ships and submarines took it up from the 1910s, then locomotives, buses, trucks and heavy equipment. The first diesel lorry appeared in 1908, the year Diesel's patents expired, and diesel passenger cars arrived slowly from the 1930s. The 1970s energy crisis, with its demand for better fuel economy, made them mainstream.
Why do diesel and petrol exhausts differ?
Because lean combustion produces two things a petrol engine's catalyst can handle easily and a diesel's cannot. The first is nitrogen oxides, or NOx. Three-way catalytic converters, which are very effective on petrol exhaust, do not work well on diesel, so a diesel emits 20 times more NOx unless it is treated. Regulators in the United States and Europe have tightened passenger-car NOx standards steadily since the early 2000s.
The second is soot. Incomplete combustion produces black carbon particles, including nanoparticles smaller than one micrometre, which worsen air pollution and are harmful to health. Two-stroke diesels make more particulate per unit of power than four-stroke ones. Cleaning up both is costly. Urea-based selective catalytic reduction cuts NOx well, but it is costly, high-maintenance and bulky.
Some figures show the stakes. A 2015 report in Nature found that one in five (20 per cent) people living in European cities breathed unhealthy levels of nitrogen dioxide. In London, diesel traffic produced 40 per cent of NOx, and air pollution there was linked to 3,000 deaths a year. The UK had also moved to a CO2-based road tax that favoured diesel, which a Channel 4 documentary called the "great car con" and which MP Barry Gardiner described as a mistake.
What are the common diesel particulate filter problems?
A diesel particulate filter, or DPF, traps soot from the exhaust. Wall-flow filters usually remove 85 per cent or more, and new ones capture from 30 per cent to above 95 per cent of the harmful soot, which can bring emissions down to 0.001 g/km or less. Filters have been used on non-road machines since 1980 and in cars since 1985, and PSA Peugeot Citroen made them standard on passenger cars in 2000. Euro 5, fully in force from January 2011, requires every diesel car to have one.
The trouble is that a filter fills up. Soot must be burned off in a process called regeneration. Passive regeneration happens on its own when the engine runs hot enough. Active regeneration injects extra fuel into the exhaust to raise the temperature. If the filter gets too clogged, a forced regeneration is started by a dashboard switch or by a garage with a scan tool.
City driving is the weak spot. Regeneration needs road speeds higher than most urban streets allow, typically a drive at 80-100 km/h for 30 to 45 minutes every few hundred km of city use. A driver who ignores the warning light and never exceeds 60 km/h risks a filter that cannot clear itself and has to be replaced. Some newer engines can run a parked regeneration by raising the speed to around 1400 rpm.
Other things can go wrong. Ash from oil use builds up and will not burn away, so filters need periodic cleaning, done carefully to avoid damage. Failing injectors or turbochargers can contaminate the filter with raw diesel or oil. Soot build-up raises back pressure and causes engine problems. The ceramic matters: the common cordierite filter is cheap and effective but has a melting point of about 1200 degrees Celsius and has been known to melt during regeneration, while silicon carbide survives to 2700 degrees but is costlier and harder to package.
Petrol vs diesel at a glance
| Feature | Petrol | Diesel |
|---|---|---|
| Ignition | Spark plug | Compression of air |
| Compression ratio | Typically lower | Higher |
| Thermodynamic efficiency | About 20-30% | Up to 55% in low-speed marine units |
| Fuel use and CO2 | Higher | Lower |
| NOx | Handled by a three-way catalyst | 20 times higher unless treated |
| Soot | Not addressed by the Euro 5 filter rule | Trapped by a particulate filter |
| Filter requirement | None under that rule | Compulsory on diesel cars from January 2011 under Euro 5 |
| First practical engine | 1876, Otto and Langen | First ignition 1893, first commercial engine 1898 |
| Typical US use | Mainstream passenger cars | Largely larger on-road and off-road vehicles |
Dieselgate explained
In September 2015 the US Environmental Protection Agency told Volkswagen it had violated the Clean Air Act. The company had programmed turbocharged direct-injection diesels to run their emissions controls only during laboratory tests, so the cars met US limits on paper while emitting up to 40 times more NOx on the road. The software went into about 11 million cars worldwide, 500,000 of them in the United States, in model years 2009 to 2015.
The discovery began with an ordinary test. In 2013 the International Council on Clean Transportation paid West Virginia University US$70,000 to test US diesels on the road, expecting them to pass. Researchers drove a Volkswagen Passat, a Volkswagen Jetta and a BMW X5 in California. The BMW was at or below the standard apart from rural hill driving, but the Jetta exceeded US limits by a factor of 15 to 35 and the Passat by 5 to 20. The Passat had even driven from Los Angeles to Seattle and back, over 3,200 km. The findings were published in May 2014.
The roots went back further. Volkswagen licensed Mercedes' BlueTec urea system in 2005, then cancelled the deal in 2007 to use its own cheaper lean NOx trap. That system needed a fuel-rich exhaust, so fuel economy suffered. Around 2006, the company programmed the control unit of its EA 189 engine to switch into a clean mode when it detected a test. Der Spiegel reported in 2015 that at least 30 managers knew for years, which Volkswagen denied. The company later pleaded guilty to criminal charges, and a statement of facts said management had asked engineers to build the defeat devices because the diesels could not otherwise pass US tests.
The bill was enormous. Volkswagen set aside 16.2 billion euros in April 2016, agreed in June 2016 to pay up to $14.7 billion to settle US civil charges, and took a $2.8 billion criminal fine in April 2017. Its share price fell by a third within days, chief executive Martin Winterkorn resigned and was charged in the United States on 3 May 2018. By 1 June 2020 the scandal had cost $33.3 billion. The affected cars stayed legal to drive in both the United States and the European Union.
The fallout spread beyond one company. A study by the ICCT and ADAC showed the biggest gaps between lab and road from Volvo, Renault, Jeep, Hyundai, Citroen and Fiat. Warning signs had existed since at least 2011, when the European Commission's Joint Research Centre found average on-road NOx of 0.93 g/km from diesels it tested and 0.62 g/km from Euro 5 diesels, far above the limits. The US limit of 0.043 g/km is among the strictest in the world, against 0.18 g/km for Euro 5 and 0.08 g/km for Euro 6.
Is diesel being banned?
Not as a blanket rule. What exists is a patchwork aimed at dirty diesels. Euro 5 made filters compulsory on diesel cars, and Milan's 2008 Ecopass levied a hefty entrance charge on diesel vehicles without a filter. Tokyo banned trucks without filters from the city limits in 2002, and London's low emission zone began charging non-compliant vehicles in 2008. The UK tightened its MOT test in 2018 to demand a working filter on diesel cars, and driving without one could draw a fine of 1,000 pounds.
California went furthest for heavy vehicles. A rule from 2008 required on-road diesel trucks and buses to cut particulate emissions by at least 85 per cent. One assessment holds that, with regulations tightening, eventually all on-road diesel engines will be fitted with filters.
What should a buyer weigh?
Engineering gives a few clear pointers. A car that mostly makes short urban trips will rarely reach the speeds a diesel filter needs, which makes a petrol engine the simpler choice. Long, steady runs suit diesel better, because the filter clears itself and the engine's efficiency counts. Anyone shopping used should ask about filter condition, and whether warning lights were ever ignored.
Prices, taxes and resale values differ by market, as do the city restrictions where you live or plan to drive.
Where this goes next
The direction is tighter control rather than abolition. NOx standards have tightened since the early 2000s, Euro 6 sits at 0.08 g/km, and the UK has added filter checks to its MOT. Diesel is also spreading in other fields: more than 5,000 aircraft diesel engines were delivered worldwide between 2002 and 2018.
