Tea Break: Compression ignition engine
Rob Marshall looks at how diesel became an engine of the future and charts its apparent fall from grace.
The history of the diesel engine.
While Rudolf Diesel learned much about heat engines from his tutor at the Technical University in Munich, who was the founder of refrigeration engineering, it is rumoured that a Victorian cigar lighter igniting tinder by compression influenced Diesel to invent the engine that bears his name.
Reducing running costs
The main difference between Rudolf Diesel’s compression-ignition piston engine over the spark-ignition design was that only air was admitted into the cylinder, prior to being compressed so that the resultant increase in temperature would ignite its fuel spontaneously. Yet, the higher compression ratios made the engine far more dangerous and it is believed that an exploding prototype nearly killed him. The main appeal was the engine’s ability to run more efficiently on cheaper fuels, which ranged from coal dust to peanut oil.
Diesel’s first compression engine patent was taken out in 1892 but the basic design was refined continually and found favour quickly in large industrial applications. By 1900, Diesel had become a millionaire but his financial affairs descended rapidly into debt. He boarded a night boat to England, en-route to a business meeting with the British Admiralty – he never arrived and, while a number of theories exist, suicide has become the most probable reason.
Rise and fall
Early diesel engines were too large and cumbersome initially to be used for popular passenger cars, although they
powered ships and locomotives. The high compressions meant that the engine had to be considerably more robust to survive the increased mechanical stress, plus they could not achieve comparable RPMS and horsepower figures of equivalent-sized spark-ignition engines, despite offering superior torque. As diesel engines became compact enough to be used on lorries and passenger taxis, they were favoured by owners that prioritised fuel economy and mechanical longevity over refinement, panache and power. These compromises could not last forever.
While some European diesel-powered passenger cars were introduced in the post-war period, most private motorists did not take diesel engines seriously until the 1970s. A major improvement came with Sir Harry Ricardo’s devising of the indirect-injection system (albeit in the 1930s) where fuel was injected into a separate swirl chamber within the cylinderhead, which helped to control the violent combustion, enhancing refinement. Indirect-injection provided passenger diesel cars with a degree of smoothness, but their performance tended to be leisurely, until appropriate power hikes were provided by turbocharging, which started to become mainstream in the 1980s. At this point, diesel was starting to be taken seriously, helped by the glamour of some models rivalling petrol-powered alternatives in the performance stakes.
The inefficiencies of indirect-injection were remedied by relocating the fuel injector back in the combustion chamber, although the actual combustion takes place in a ‘bowl’, cast in the piston crown. Fiat’s otherwise unremarkable Croma model of 1986 is credited as being the first conventional passenger car to feature direct-injection, yet refinement was sacrificed for enhanced fuel efficiency. These direct-injection downsides were addressed by advanced electronics and a hollow metal tube storing diesel at high pressure, which is connected to electronically-actuated fuel injectors.
By metering several small amounts of fuel, prior to the main combustion injection, smoother combustion could be promoted and this ‘common-rail’ system was key to bringing refinement to direct-injection diesels. Pioneered by Magneti Marelli and Bosch, commonrail fuel injection debuted in the 1.9-litre Alfa Romeo 156 JTD. Naturally, precise fuelling control meant that fuel injectors had to become more sophisticated. Aside from conventional solenoid-activated types, Piezo fuel injectors employ an internal piezoelectric crystal that expands when electrified. A downside is that most types cannot be reconditioned. In any case, such advances in diesel technology saw them become accepted widely by the general public by the end of the 1990s.
Emissions take over
Despite diesel engines attracting favourable road taxation rates throughout Europe for their low CO2 emissions, relative to petrol, Euro emissions standards dictated increasingly stringent caps on soot/ particulate and NOX emissions from the beginning of the 21st century. PSA Peugeot Citroën introduced the first Diesel Particulate Filter (DPF/FAP) in 1999 on the direct-injected 2.2-litre HDI engine of the Peugeot 607. DPFS became legal requirements on all new cars from Euro 5 (2009) but many short distance drivers experienced issues with DPFS blocking, which tainted diesel’s reputation for reliability and started to limit its almost unending appeal.
NOX emissions were recognised as being a particular issue for diesels and Exhaust Gas Recirculation (EGR) valves were fitted to reduce the combustion temperatures. This created a problem, because more particulates would be created that the DPF would have to resolve. Yet, EGR was insufficient by the time Euro 6 emissions legislation came into effect in 2014. Most manufacturers adopted SCR technology, which injects a distilled water and urea solution (‘Adblue’) into a special catalyst, located in the exhaust system, to treat the gases prior to them entering the DPF. Most modern diesels are fitted with separate low- and high-pressure EGR circuits; the low-pressure system speeds engine warming, while the cooled high-pressure system extracts exhaust gases from the DPF outlet, which reduces the risk of carbon contamination in the inlet that affected earlier Egr-equipped diesels.
Despite all of these technical developments, it is clear that the ambitions of politicians were not shared by carmakers, many of whom adapted their engines to ‘learn’ official exhaust emissions test cycles and produce tailpipe outputs that were not relevant to realworld operation. The resultant ‘diesel-gate’ scandal of 2015 tainted diesel’s image, leading some carmakers to announce that they were ceasing development, or abandoning the fuel altogether, in favour of hybrid petrol/electric vehicles. However, since there is no alternative fuel that offers equivalent economy and performance, some car manufacturers have stated that diesel-engined models will remain an important part of their model mixes, in the medium-term at least.