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Diesel Engines: What's the truth?

Until as recently as a few years ago, diesel cars were heralded as the more environmentally friendly option to drive. When they became subject to considerable negative publicity – exaggerated in no small part by the Volkswagen scandal – people who had bought diesel cars were unsurprisingly disappointed. Had they been misled about the amount of emissions the engines produced? Are the rumours about the government planning to discourage people using diesel cars, or banning them from urban areas, true?

This guide will outline the truth about diesel engines, and aims to dispel common myths.

A brief history

The promotion of Diesel

Back in 1994, only 7.4% of cars in the UK were diesel. Fast-forward over a decade, and by December 2016, there were 12.1 million diesel cars, accounting for 39% of the total vehicles on the UK’s roads.

But what encouraged people to drive diesel cars? Well, in the early 2000s, the government changed road tax to a CO2-based system which favoured diesel cars as they generally emit less CO2 than petrol counterparts. In other words, the government threw its weight behind the sector. As such, British car makers heavily invested in the necessary manufacturing processes and flooded the market with suitable diesel cars.

It wasn’t all about reducing your car tax bill, though. Diesels were promoted as the environmentally-friendly choice because they use less fuel and more air to get the same performance as a petrol engine. This ‘lean-burn’ was a huge selling point.

CO2 emissions - On average, petrol emits around 200g CO₂/km. For diesel, the average is around 120g CO₂/km.

Source: The Engineer

What went wrong?

Criticisms started emerging that the government had focused on just one area – to reduce greenhouse gas emissions by minimising CO2 output – and ignored other impacts of diesel. In reality, other emissions include:

  • Particulate matter (PM)
  • Nitrogen oxides (NOₓ):
    • Nitrogen dioxide (NO₂)
    • Greenhouse gas nitrous oxide (N₂O)
    • Nitric oxide (NO)

Understandably, diesel car owners were frustrated. They were now suddenly being told their cars were producing fumes that could affect local air quality.

The expert reaction

In response to growing criticism, the car industry stood its ground. In March 2015, a campaign launched by the Society of Motor Manufacturers and Traders (SMMT), sought to “challenge the increasing demonisation of diesel” vehicles. They promoted the credentials of the new Europe-wide Euro-6 standards, which came into force in September 2015. It forced car makers to limit NOx emissions from new diesel cars to 80mg/km.

The limits are designed to bring down air pollution levels in cities across Europe. The car industry was clearly showing their commitment to local air quality, as well as continued efforts to slow down the pace of climate change. They weren’t just looking at one element; the industry had an all-round view of the impact of diesel and were dedicated to future-proofing change.

They also honed in on the reality that technology is catching up, with numerous successes for car manufacturers in reducing emissions of NOx, particulate matter (PM) and CO2. For instance, research has shown particulate filters in-car exhausts can reduce PM emissions by more than 90%. They’re now a legal requirement on all diesel cars registered since 2009.

The industry continues to prove itself too, with new diesel cars, built to the latest standards. To maximise efficiency, they require good operating conditions and regular maintenance. How did consumers respond? The numbers speak for themselves. In 2016, a record 1.3 million new diesel cars were registered in the UK – that’s up 0.6% on the previous year. The trend looks set to continue in 2017, with buyers valuing the high performance and low fuel consumption of diesels.

What's the recent bad press about?

Although diesel continues to dominate fuel consumption in the UK (see table), there have been a few rough patches – notably the Volkswagen scandal.

UK fuel consumption by type Scroll to view this table
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015
Petrol 21.4 21.9 20.8 19.9 19.5 18.9 18.1 17.6 16.5 15.6 14.6 13.9 13.2 12.6 12.3 12.1
Diesel 15.6 16.1 16.9 17.7 18.5 19.4 20.2 21.0 20.5 20.1 20.7 21.0 21.5 21.9 22.7 23.7

In 2015, it emerged the company had falsified emissions data on its diesel vehicles. They’d been installing software – known as ‘defeat devices’ – to pretend the cars were cleaner than they are. 1.2 million of these devices were installed in the UK.

It turned out the software enabled cars to detect when they were under test conditions. As such, they’d deploy full pollution controls – which were, crucially, not used in normal driving conditions – to meet the required standards. In reality, Volkswagen cars were pumping out up to 40 times the permitted level of pollution.

As a manufacturer that fights with Toyota for top spot as the world’s biggest car company, it was a real shock for the industry. Not only did it send the company’s share price plunging, but it damaged trust on a wider scale, especially as the firm was amongst those running its own marketing campaign in favour of clean diesel.

It was exactly what diesel didn’t need. The emissions scandal leaked damage across an industry trying to clean up its reputation, making it more important than ever to dispel common myths.

Common myths about Diesel cars

Myth: Diesel is the largest cause of air pollution

Reality: There are large misconceptions, especially about diesel’s contribution to air pollution

In total, cars only make up 14% of the nation’s overall NOx emissions, according to the SMMT. But the negative press has obviously had an impact on the public, as more than half of those surveyed by YouGov incorrectly identified cars and commercial vehicles as the biggest cause of air pollution in the UK.

In fact, the Diesel Facts website says: “It would take 42m Euro-6 diesel cars (almost four times the number on the roads) to generate the same amount of NOx as one UK coal-fired power station.”

It just isn’t true that diesel cars are the main source of urban NOx. Even in London, where road transport as a whole is responsible for around half of London’s NOx, diesel cars produce just 11% (this varies depending on congestion levels).

Myth: You can't achieve the stats in real life

Reality: Real progress has been made in closing the gap between test conditions and real results

The VW scandal made it harder to trust the emissions and fuel consumption figures advertised by car manufacturers. But regulations will be tighter as a result. All new cars must meet Euro 6 standards for exhaust emissions of NOx and other pollutants.

For diesels, the permitted level of NOx has dropped from 180mg/km (Euro 5 emissions standards) to just 80mg/km.

And the results are starting to speak for themselves. SMMT reports real world tests using the London 159 bus route show a 95% drop in NOx compared with previous generation Euro 5 buses. If every older bus operating in the capital were replaced with a Euro 6 version, total NOx emissions in London would fall by 7.5%.

Progress continues too. In September this year, a new official EU-wide emissions testing system will come into force. For the first time ever, on-road testing will be used to reflect the varied conditions involved in real-world driving. For car makers, they’ve got to invest money to ensure their cars match the high standards.

Things like speed, congestion, road conditions and driving style will be measured to get a more accurate view of whether drivers would be able to achieve the figures in real life. It’s a clear signal the industry is moving in the right direction.

Myth: Diesel is dying out

Reality: Consumers are recognising the benefits of new diesel cars

Are diesel cars on the way out? 360,000 diesel cars were sold between January and March this year, almost 90,000 more than the same period in 2012. They’re not going anywhere. Buyers are clearly recognising the technological advances which have reduced emissions enormously in recent years:

  • Carbon monoxide (CO), petrol down 63%; diesel down 82% since 1993
  • Hydrocarbons (HC), petrol down 50% since 2001
  • Oxides of nitrogen (NOx) down 84% since 2001
  • Particulate matter (PM), diesel down 96% since 1993
Source: SMMT

Myth: Governments are clamping own on diesel cars

Reality: It's complicated

Governments have tough criteria to meet. But the real issue is with older diesels. They’re not out to target people buying new cars. Some examples from around Europe include:

  • Berlin has banned the oldest, highest-polluting diesel cars from its city centre
  • Munich’s clean air ban will bring in some form of diesel ban in 2018
  • Madrid has introduced a system to cut the number of cars on the roads in half during smog outbreaks
  • Oslo city council plans to raise the road toll for diesel cars entering the city centre from 33 Krone (£3) to 58 Krone (£5.50) in rush hour

Back in the UK, in October 2018, diesel vehicles made before 2005-06 will have to pay a £10-a-day toxicity charge in London. That’s on top of the existing £11.50 congestion charge. No cities are currently implementing measures to cut down on the number of newer diesel cars.

Myth: Drivers don't care about pollution

Reality: Awareness is growing, as is a desire to support change

Drivers are increasingly concerned about air quality. According to RAC research, motorists are keen to play their part in bringing down pollution levels. Key findings include:

  • 66% of drivers said they’d support strong action from the Government aimed at tackling air pollution
  • 55% said they’d be in favour of ministers levying charges on the dirtiest vehicles when they enter polluted areas
  • 57% would not object to drivers of older diesel vehicles being required to pay charges when entering town and city centres

How much could you save? On average, diesel cars are more efficient by about 8mpg (based on tests of 249 cars that all meet the latest emission limits: Euro 6). It could save you around £200 a year.

Source: Which?

A comparison of the cleanliness of new diesel vs. new petrol

Diesel vehicles are often cited as the more efficient choice of car – which basically means you can go further on the same amount of fuel. Getting more miles for your money is something that convinces many people to buy diesel cars. But are they ‘dirtier’ than their petrol counterparts? Both petrol and diesel are derived from crude oil, a mixture of hundreds of hydrocarbons. They all create emissions.

Petrol cars emit higher levels of carbon dioxide, a greenhouse gas which contributes to global warming. It is also largely accepted technological advancements in petrol engines have been slower than updates and improvements to diesel engine technology. But the results achieved when you buy and drive your own car largely depend on how you plan to use it. To highlight, let’s look at some of the best-selling cars in the UK:

Petrol vs diesel: premiums and running costs Scroll to view this table
Fuel type & car Price as new Claimed MPG (& 9,000 mile fuel cost) Total one-year cost Total five-year cost Total five-year cost (with 15,000 miles a year) C02 emissions
Petrol Ford Focus (1.5T EcoBoost 150PS) £22,055 Includes first year car tax (£160) 51.4mpg (£907) £22,962 £27,150 £29,613 127g/km
Diesel Ford Focus (2.0 TDCi 150PS) £23,575 Includes first year car tax (£140) 70.6mpg (£668) £24,243 £27,475 £29,141 105g/km
Petrol Vauxhall Corsa (1.4i, 75PS ecoFLEX) £12,960 Includes first year car tax (£160) 54.3mpg (£859) £13,819 £17,815 £20,118 120g/km
Diesel Vauxhall Corsa (1.3 CDTi EcoFlex SE) £15,090 Includes first year car tax (£140) 72.4mpg (£652) £15,742 £18,910 £22,798 93g/km
Petrol Nissan Qashqai (1.2 DIG-T 115) £21,095 Includes first year car tax (£160) 50.4mpg (£925) £22,020 £26,280 £28,803 129g/km
Diesel Nissan Qashqai (1.5 dCi 110) £22,805 Includes first year car tax (£120) 74.3mpg (£635) £23,440 £26,540 £28,096 99g/km
Source: Which? / Our calculation for travelling 15,000 annual miles for five years: Cost of (1,000 miles fuel cost x 15) x 5 years (+ price as new)

Generally, calculations which help buyers to decide what car would be the more affordable choice focus on a set number of annual miles. As the Which? example shows, over five years, the price difference is close for when averaging around 9,000 miles.

Up the annual mileage to 15,000, though, and over a five-year period, the Focus and Qashqai diesels are cheaper. Owners of these cars can expect savings every year. It’s also worth looking at the CO2 emissions for those cars to exemplify the tendency of diesels to emit less.

Deciding what car is right for you

As the examples show, your ideal car depends on its usage. There’s no hard and fast rule as to which is best, as both diesel and petrol engines have their pros and cons. Deciding what car is right for you depends on how much you’ll be driving, and where. Some key questions to ask yourself:

Where do you drive most?

Motorways, A and B roads, or towns? Wide-open roads are where diesel cars will be in their element. To contrast, regular stop-start urban traffic is better-suited to petrol cars. It’s all down to how the engines work. Petrol engines rev more freely, and are good for quick acceleration. Time and time again, the engine will easily get within its efficient powerband. In contrast, stop-start traffic can clog diesel particulate filters (DPFs). However, diesel engines offer more low-speed torque so tend to have better overtaking power and towing ability than petrols.

Do you regularly drive short or long journeys?

If your yearly mileage is over 12,000 miles, you’re probably better off with a diesel.

How much do you have to spend initially, as well as expected on-going costs?

Diesels can still be cheaper to run than a petrol, but the initial costs are more.

What's your driving style like?

If you like to cruise with the knowledge you’ve got the power there when needed, diesels will live up to your expectations. They’re known for the balance of performance and efficiency, as well as driving refinement. If you enjoy extra oomph and agility, newer petrol cars could better satisfy your ‘sportier’ preferences.

Pros of diesel

  • Greater fuel efficiency
  • Lower Co2 emissions
  • Refined and powerful driving experience
  • A tendency to have higher resale values

Pros of Petrol

  • Fun to drive
  • Generally cheaper to buy
  • Lower n0x emissions

Cons of diesel

  • Older diesels produce other emissions, including the toxic NOx, a gas which contributes to the build-up of smog
  • Can cost more to buy
  • Noisier engines

Cons of Petrol

  • Lower efficiency
  • Higher Co2 emissions

How is technology reducing the amount of emissions newer diesels create?

When you’re buying a new diesel, you’ll hear about lots of technology. Here’s what the key elements do to improve the environmental impact of diesels:

  • NOx traps include absorbers to reduce the emissions from the exhaust. Think of them like molecular sponges. They’re continually being improved, but need to be replaced to maximise efficiency.
  • Diesel particulate filters (DPF). Fitted in the exhaust, these filters stop engine soot passing into the local air. To keep them working, though, they should be emptied. This happens passively when the exhaust temperature is high enough. That’s why it’s important to regularly drive diesels on motorways or fast A-roads.
  • Exhaust gas recirculation (EGR) takes a portion of an engine's exhaust gas and forces it back to the engine cylinders. That means there is less oxygen rich air available for combustion, consequently reducing the formation of NOx.
  • Selective catalytic reduction (SCR). Best described as an exhaust after-treatment technology, it uses a catalyst to break down NOx. You must top your car up with AdBlue to keep it effective.

As a result of such developments, it would take 50 new cars today to produce the same amount of pollutant emissions as one vehicle built in 1970.

What does the future hold for diesel?

Diesels sell. They’ve always made – and continue to make – sense for many drivers. Those who are covering a lot of miles every year prefer the balance of power, smoothness and efficiency.

But if you’ve ever felt tricked by the environmentally-friendly diesel dream, it’s understandable. Cracks did appear in what manufacturers and governments were telling buyers. But they’ve learnt their lesson, and diesel efficiency has improved as a result. Things are only getting better, and that advancement now needs as much attention as Dieselgate. As Mike Hawes, SMMT Chief Executive, says:

“Euro 6 diesel cars on sale today are the cleanest in history. Not only have they drastically reduced or banished particulates, sulphur and carbon monoxide but they also emit vastly lower NOx than their older counterparts – a fact recognised by London in their exemption from the Ultra Low Emission Zone that will come into force in 2019.

“Some recent reports have failed to differentiate between these much cleaner cars and vehicles of the past. This is unfair and dismissive of progress made.”

The images of smog-choked streets can be a distant memory; diesel has now progressed light-years away from the old technology. Even the scary NOx emissions can be converted into harmless nitrogen and water before it reaches the exhaust.

With the arrival of a new official EU-wide emissions test, due this year, further improvements can be expected. SMMT call it the world’s toughest-ever emissions standard, as it will encourage manufacturers to focus on real-world driving results. People will be able to trust in the figures advertised by car makers, and feel confident they’ll be able to achieve similar in varied conditions. Without a doubt, it’s a move in the right direction.

What’s more, as efficiency continues to improve – driven by tough testing and consumer demand – diesel will be a contender for many years to come.

Things might change. CO2 – which has long been the biggest focus for European emissions regulations – is no longer the sole focus. But diesel has proved it can keep up.

A welcome development in the car industry is the growing presence of alternatively-fuelled vehicles, with year-on-year sales up 31% in March 2017. Conventional hybrids, plug-in hybrids and electric vehicles are quickly becoming a more realistic choice of car for more people.

Supported by government incentives, technology improvements (notably to the range), and more charging points, the future looks like one typified by choice for car buyers. Petrol, diesel or alternatively fuelled, arm yourself with the right information and pick a car that’s right for you.