Passenger car in the United States never had luck with diesel engines and the last attempt to change the tide resulted to what it seems a global reverse of enthusiasm towards this kind of propulsion. Is compression combustion passenger car really in a death row at this point with the gas engine following closely and electric vehicles stepping up to the throne? Or maybe not so fast.
It’s hard to imagine the press for diesel to be worse these days. More and more towns and cities set deadlines after which cars using this kind of engine won’t be allowed. Car manufacturers follow with statements on how they stop developing the technology, they will do it in the near future or have already set the date for the last diesel call to roll out of the assembly line. How did we get here so quickly from the peak of interest in this highly efficient technology during the times when the price of a barrel of crude oil was flirting with $150 level?
Diesels never managed to get a lot of traction with just 4% share in the US market of passenger cars and light trucks according to the Department of Transportation data https://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/bts_fact_sheets/oct_2015/html/figure_01.html. Even with gas price reaching past $4 per gallon more than once in the past 10 years the change of interests among car buyers turned more into hybrid and smaller gas engines than diesels. Europe car market in the contrary is a whole different animal with the compression combustion engine car figures often exceeding gas powered vehicle sales according to Eurostat http://ec.europa.eu/eurostat/statistics-explained/index.php/Passenger_cars_in_the_EU data. This ratio was pretty much the same through the modern automobiles ages. Let’s take a look td the factors contributing to such a discrepancy between US and European markets.
Consumer decision process in passengers car consists of two components, practical and emotional, both contributed to uneven diesel distribution among the markets. United States drivers enjoy relatively inexpensive fuels especially when compared to their European counterparts mainly due to taxation level. US government imposes federal and state taxes for gas and diesel; while the federal tax is fixed the state one varies so the price level is different between the states. Still the tax component of the fuel price hardly ever goes over 25%. In Europe the situation is quite different; a combination of different tax fees apply to various fuels. VAT (value added tax), fuel tax, excise (luxury goods tax) and recently more often some sort of emission fee applies to each car fuel. In result it’s not unusual for taxes to contribute to over 50% of the final price of the liquid fuel for the internal combustion engines. In order for some of the European industry branches to stay competitive special tax breaks apply to certain diesel usage like for example in United Kingdom agriculture and construction equipment use far less taxed fuel. This kind of diesel is colored with red dye in order to mitigate the risk of tax fraud schemes. The bottom line is that when compared to the US the gas station prices in Europe needs to be multiplied by 2 to even 3 times depending on the country. As shocking as it may sound for those who never had to buy gas or diesel in Europe the fact of the fuels being more expensive alone doesn’t explain the great difference in compression combustion engine application advantage in Europe. In order to explain diesel augmented share we need to dig a little deeper into technology of heat engines. Each internal combustion engine regardless of fuel type transforms heat energy into the kinetic one but that happens with losses as it’s impossible to transfer the entire heat produced by burning hydrocarbon fuel into movement energy. Some of the energy remains in the form of heat. Current diesel engines efficiency can reach up to 50 percent while gasoline power plants have hard time reaching 40%. Engine efficiency is the key in understanding diesel strong presence in European car market. While diesel fuel might be just as expensive as gasoline lower quantities are used to generate the same amount of power resulting in better overall economy.
Diesel engines are just as efficient in the US as anywhere in the world one may say. This is true but when combined with higher vehicle sticker price (diesels are more expensive to manufacture due to engine mechanics complication level difference) the lower fuel price is not enough to offset that difference as it in Europe. Moreover car purchase decision is almost never entirely rational process and diesels are anything but romantic or sexy. The loud clunking noise at idle and up to around mid 2000s puffs of thick, black smoke upon cold start or under heavy load cannot be considered as favorable contributing factors towards purchasing a motor vehicle in passenger segment. Design engineers have never given up the idea of bringing compression ignition power plant to compete head to head with its gasoline counterpart.
Common rail system was a new chapter for the Diesel engines that unfolded in the 1990s and become standard in 2000s. The basic concept of pumping fuel separately for each injection event was replaced with a solution providing highly available fuel under high pressure. Having fuel under pressure available at all times an engine can take advantage of using multiple injections during a single work cycle. This change has proven to be revolutionary for the compression ignition engine. The loud knock sound coming from the older diesels takes its origin from the fact that the fuel mixture ignition happens at a very rapid pace. In order to address that issue the passenger car manufactures tried various solution including a pre-combustion chamber between the injector and combustion chamber but each of these solutions had side effect such as poor performance and difficult cold start during winters. Having common rail on your side the solution become simple all of the sudden, a pilot portion of fuel injected prior to the main phase caused the ignition process to be far less violent resulting in significant noise level reduction without compromising performance. Moreover noise reduction was just the beginning of the common rail revolution. Next came the turbocharger which makes diesel engines effectively supercharged since the direct fuel injection into the combustion chamber was in place already; which naturally results in high performance specifications. Next came the visual tuning handled by a diesel particulate filter or DPF for short. Soot is the primary factor responsible for the thick black smoke in compression ignition engine exhaust fumes. Around 2005 DPF became standard in most diesel cars virtually eliminating the ugly black smoke from the image. These filters operate in a fairly simple fashion in most cars. A set of tiny screens catch soot particles separating it from the combustion produced smoke. Once certain level of soot accumulation in the filter is reached the computer running the engine allows for higher exhaust temperature which allows for DPF regeneration once the soot burn rate is achieved. This simple scheme has some weak points however. Soot burn heat is hard to achieve in idle RPM levels or during bumper to bumper traffic. The driver has only so much time to get the engine to the right power level once the DPF maintenance control light illuminates. If the DPF regeneration doesn’t occur within a specified period of time a permanent damage to the filter occurs resulting in a need for costly repair.
It felt like by mid 2000 diesel might had it all, the performance, noise, emission and image under control. It certainly did, except the standards have changed quite a bit. All these great improvements addressed the issues as we knew them in 1980. 2005 was whole another world. By 1990 it was widely known that nitrogen oxides, particularly nitric oxide and nitrogen dioxide have devastating effect on the atmosphere and air pollution. Often referred as NOx, nitrogen oxides have capacity of causing acid rains and forming nitric acid in reaction with ammonia. These highly toxic chemicals have capacity to react with wide variety of other elements creating mixtures hurting lungs and deforming DNA.
NOx can’t form in ambient temperatures and the only way they form naturally is during a lightning strike when high temperature and high pressure conditions occur briefly. While lightings may feel as common meteorological phenomenon they can’t be considered as a major contributor to the NOx present in the lower parts of the atmosphere wherey they matter for organic life health. Scientists led by Lesley E. Ott have estimated annual lighting NOx contribution at 8.6 million tons while combustion emissions resulting from fossil fuel combustion are estimated at 28.5 million tons http://onlinelibrary.wiley.com/doi/10.1029/2009JD011880/pdf. While it may seem that the lightings are responsible for nearly 25% of the NOx production it needs to be noted that over 90% of the lighting formed NOx remain 15 thousand feet above Earth surface and all combustion related NOx stay close to the ground.
NOx issue is another example when the same problem is far easier to solve when it comes to gasoline engine as opposed to compression combustion one. Three-way catalytic converter introduced widely in early 1990s helped reduce NOx in the gasoline engine exhaust gases by reducing NOx to nitrogen and oxygen. Unfortunately for diesel engines the same recipe cannot be used due to the fact that diesel exhaust gasses have much higher oxygen content comparing to its gasoline counterpart. Therefore any potential NOx reductant would first react with oxygen. Another way of reducing NOx had to be found. Chemical scientist engineers offered 2 potential ways of dealing with the problem. One was injection of urea into exhaust gases which helped to reduce NOx to nitrogen and water. Multiple patented urea solutions have been developed, AdBlue and BlueTec would be the examples of this approach. The other solution is the exhaust gases recirculation valve, commonly referred as EGR. Used for a while in spark combustion engine found its way to help reduce NOx levels in diesels. Since diesel operate under abundant air conditions (there are no air throttles in diesel engines) EGR helps reduce the peak ignition temperature by reintroducing some of the exhaust gases back into the combustion chamber. This solution doesn’t require urea tank and refills but has limitations of its own. Reduced combustion temperature during the power stroke means power decrease. It further reduces power by creating back pressure during exhaust stroke. Additionally exhaust gas recirculation doesn’t help engine lubrication which increases risk of premature component failure. VW emission scandal has shown just how difficult the NOx reduction could be.
It broke out from the best intentions of West Virginia University scientists trying to understand how German engineers managed to develop clean diesel technology which could help fuel economy of cars in the United States. Keep in mind it was 2014, when gas station fuel prices over $4 per gallon was considered normal and every way to reduce consumption could be worth exploring. Unfortunately, no miraculous solution was found and VW admitted in September 2015 to introduce test cheating software. It seems like diesel future has been sealed back then; in passenger cars anyway. While the emission scandal involved relatively low number of vehicles the bad world has spread on all compression combustion engines. The bottom line is that it’s very hard for diesel engine to provide great performance, fuel economy and meet strict emission standards at the same time. Even though clean diesels do exist, remain majority the term itself sounds more like an insult these days than a catchy marketing slogan. Nobody driver wants to wear emission cheater id badge.
Declaration of manufacturers to stop developing diesels and cease production by this or that year combined with announcements that many towns won’t allow diesel cars can’t help encourage prospective buyers. Is this the end of compression combustion engine? I’m always careful to support such big words as the fate of each solution depends on many factors. Sure, the time to market diesels couldn’t be worse but dust always settles eventually even after the biggest storm. What kept diesels alive for very long time, which is better efficiency ratio vs gasoline engine is not going anywhere. Electric cars if they prevail will eradicate both internal combustion engine types but if they fail many would look back for another efficient solution and I wouldn’t be surprised by a cool headed conclusion there was nothing wrong with the diesel to begin with, you just need to develop it one step a time.