In diesel engines exhaust particulates and NOx can be controlled by promoting premixing of the fuel with air and combustion at a low temperature. However, conventional diesel fuel is very prone to auto-ignition and will ignite soon after injection starts, before the fuel and air have had a chance to mix. Indeed most of the advanced technology used in diesel combustion systems is aimed at countering the low ignition delay of diesel fuel either by slowing down the chemistry or by enhancing mixing. Even then low-smoke, low-NOx combustion is only possible at low loads. Engine experiments show that fuels that are more resistant to autoignition, such as gasoline, improve such combustion processes very significantly. These findings have important implications for fuels for future CI engines
We define diesel fuels here only in terms of their auto-ignition quality, as those with Cetane Number (CN) > ~ 30 and gasoline fuels as those with CN < ~30 or Research Octane Number (RON) > ~ 60 [1]. In practice, diesel fuels are significantly less volatile than gasoline fuels [1]. Auto-ignition is determined by chemical kinetics [2], which depend on the pressure and temperature development, mixture strength and chemical composition of the fuel. Typical, practical diesel fuels have CN > ~40 and will auto-ignite very quickly before the fuel and air are mixed sufficiently. If the mixing is accelerated or if the chemical reaction is slowed down, autoignition can be made to occur after the fuel and air are better mixed and soot levels can be reduced. NOx levels can be reduced by reducing combustion temperature by either running lean, pre-mixed or using EGR, exhaust gas recirculation.
Several operating strategies have been employed to promote pre-mixed and low-temperature combustion in CI engines. In homogeneous-charge compression-ignition (HCCI) the fuel and air are fully premixed as in a spark ignition engine and compressed till the charge auto-ignites. In modern direct-injection (DI) diesel engines with common rail injection systems, HCCI combustion can be obtained by injecting the diesel fuel very early in the cycle, giving it enough time to mix completely with air. However, with HCCI, there is no in-cycle control over the phasing of the heat release, which will be determined by the initial conditions of the mixture at the start of the compression stroke and the auto-ignition characteristics of the fuel. Hence HCCI combustion is very difficult to control. Practical CI engines always need fuel injection near top-dead-centre (TDC) to control combustion phasing.
However, when a conventional diesel fuel is injected near TDC, it ignites very soon after injection starts. Most of the fuel at higher loads is injected after the start of combustion and burns in a diffusion flame. In fact, most of the high technology used in advanced diesel engines, which makes them complicated and expensive, is required by the necessity to overcome the propensity of diesel fuel to auto-ignite easily in order to promote premixed combustion. Thus high injection pressures and high swirl are used to increase mixing rates while high levels of cooled EGR are used to delay combustion; this in turn requires higher boost pressures to achieve the required loads. Even then, with conventional diesel fuels, low NOx and low smoke with partially premixed CI combustion is possible only at low loads. Modern engines now require after-treatment systems to further reduce NOx and particulates, making them even more complicated and expensive.
Fuel auto-ignition quality effects on partially premixed diesel combustion have recently been studied in two different single cylinder research engines – one with a displacement of 2 litres [3,4] and a compression ratio (CR) of 14 and another with a displacement of 0.54 litres and CR of 16 [5].
Both these engines could be run on gasoline-like fuels. If gasoline is injected near (but before) TDC, it ignites much later than diesel fuel and combustion occurs when fuel and air have had more chance to mix [3,4,5]. If the same amount of gasoline is injected early at the same conditions i.e. with fully premixed, HCCI conditions, ignition might not occur at all. Thus the inhomogeneity is essen- RON gasoline. However, for gasoline, even at tial for combustion to occur but the high ignition around 12 bar IMEP, FSN is below 0.1 (Figure 3). delay makes combustion happen when fuel and air Increasing the EGR level to ~41% brings down the are better mixed – fuel and air are “premixed NOx significantly for gasoline without really inenough‖ but must not be fully premixed. This mix- creasing the smoke. Of course such an increase in ture stratification, which enables control of the EGR level would increase smoke even above the phasing of combustion through injection timing, is levels shown in Figure 3 for diesel fuel. easily achieved simply by relatively late (compared to HCCI) injection. Much higher loads can be achieved at low smoke and NOx levels with gasoline compared to diesel while retaining control over combustion. At low loads, when the global mixture strength is lean, the higher the ignition delay, the leaner are the mixture packets where heat release occurs by auto-ignition and NOx is reduced very significantly [3,5] and the maximum heat release rate and hence the maximum pressure rise rate are also reduced [5]. This is illustrated in Figure 1, where Indicated Specific NOx (ISNOx) and Maximum Pressure Rise Rate (MPRR, open symbols) are both plotted against IMEP for a European Diesel fuel of 56 CN (Cetane Number) and a gasoline of 84 RON. The engine was running at 1200 RPM without any EGR and the injection timing was varied – further details about the operating conditions can be found in [5].
Specific fuel consumption is the same for gasoline as for diesel fuel if the combustion phasFig. 1: ISNOx and MPRR vs IMEP for 84 RON gasoline ing is kept the same [3,4,5]. However, HC and CO a1n2d0d0ieRsPelMfu,enlofoErGthRe, snaombeooesntg,i6n5e0obpaerraintjiencgticoonnpdriteiosn-. are higher [3,4,5], as expected for a more presure. From Fig.s 5 & 6 in [5] mixed combustion in an overall lean or dilute mixture, and would need to be controlled by exhaust As load is increased, NOx goes up even for gaso- gas after-treatment. Also, at higher loads, as the line but, unlike with the diesel fuel, NOx can be overall mixture strength becomes richer, a more reduced using EGR without really increasing the premixed burning regime will result in higher heat smoke. This is illustrated in Figures 2 and 3 where release and hence, higher pressure rise rates ISNOx and FSN smoke are plotted respectively [3,4,5]. This can be alleviated by using multiple against IMEP for the diesel fuel and a gasoline of injections [4,5]. This is usually not possible with 95 RON. In these tests the engine speed was 2000 diesel fuel - double injection makes smoke and RPM, the intake pressure was 2 bar abs. and the efficiency worse because, the first, early injection load was varied while holding the EGR level and releases heat during the compression stroke [4]. the combustion phasing fixed. As IMEP is in- Also with gasoline, provision has to be made for creased, exhaust CO2 concentration increases and starting the engine, probably via a spark plug. so does the intake CO2 level, and ISNOx decreases. With ~32% EGR the reduction in ISNOx with IMEP is similar for both the diesel fuel and 95 whether diesel fuel should be replaced by gasoFurther Work Required line. Thus should the minimum cetane specification
Thus the fuel needs to be as much like gaso- for diesel fuel be raised in those areas where it is line as possible if the aim is to promote premixed currently low (e.g. U.S.) compared to Europe? If combustion i.e. increase engine ignition delay in CI future engines are designed to promote premixed engines in order to control smoke and NOx. The combustion, higher fuel cetane number will cerextent to which this is possible in practice depends tainly not help; it might hinder such operation. On on whether other critical requirements for a practi- the other hand if the strategy used to control cal engine such as good transient and low-speed smoke and NOx at high loads is to promote (low operation, low noise, low emissions and low cost temperature) diffusion combustion and use excan be met. The engine needs to start and run at haust after treatment, higher cetane might help in low loads satisfactorily. A CI engine operating on reducing HC and CO emissions. Of course, other gasoline through the entire range is conceivable. diesel quality issues such as sulphur and volatility Such an engine could be started by using spark levels and other compositional issues are also ignition or a powerful glow plug, run with a single important in the debate about future fuel requireinjection pulse near but before TDC at low and ments. These will eventually have to be agreed by moderate loads. At high loads the fuelling rate the different stakeholders such as the auto and oil would be increased by additional injection pulses industries, governments and regulatory bodies and in the cycle. The engine will need to have an oxi- environmental interests. dation catalyst to control HC and CO emissions and a turbocharger or a supercharger. Such an In the long term, fuels much more like today’s engine would be very similar to HCCI / SI prototype gasoline rather than today’s diesel fuel could be engines that have been announced by GM and used in CI engines to great advantage. Hence Mercedes-Benz. Those engines use gasoline, are components normally considered for blending fuels started using a spark plug, run in HCCI mode at for SI engines, including biofuels like ethanol, low load with early fuel injection and switch to SI could be used to make fuels for CI engines. Indeed combustion at high load though they could run in the optimum fuel for such combustion systems CI mode at high loads as well by injecting fuel near could have a lower octane number and lower volabut before TDC. There might also be scope for tility (higher full boiling point) compared to today’s improving the efficiency of the transmission system gasolines. Such a fuel would maintain the advanfor light-duty applications by reducing the average tage over diesel fuel while easing low load, high speed since gasoline allows higher loads to be speed and high EGR operation. It might also take reached at any given speed with low NOx and low less energy to manufacture such low octane fuel. If smoke compared to diesel fuel. such a fuel is to constitute the majority of refinery
The experiments using gasoline have so far output—as a majority of the engines would be CI been done with a standard diesel injector. Small engines because of their higher efficiency—the injector holes and high injection pressures are consequences for fuel manufacturing, energy balneeded for diesel fuel in order to increase mixing ances and well-to-wheel greenhouse gas emisrates to counter the low ignition delay. These are sions need to be understood. probably not necessary for gasoline fuels with high ignition delay. There should be further scope for References optimization of this type of combustion through [1] Kalghatgi, G.T., ―Auto-ignition quality of practical injector design and optimizing injector strategies. fuels and implications for fuel requirements of future There is a lot of scope for cost saving if lower injec- SI and HCCI engines‖ SAE 2005-01-0239, 2005. tion pressures could be used. The relative impor- [2] Westbrook, C.K., ―Chemical kinetics of hydrocarbon tance of fuel volatility, composition (e.g. aromatic iinggnistioonftihnepCraocmticbaulsctioomnbInussttiitountes,yvsotle.2m8s,‖p,pPr1o5c6e3e-dcontent) and auto-ignition quality in such combus- 1577, 2000 tion systems needs to be studied further. However [3] Kalghatgi, G. Risberg, P., and Ångström H-E, ― Adresults so far suggest that auto-ignition quality is vantages of fuels with high resistance to auto-ignition by far the most important parameter. in late-injection, low-temperature, compression ignition combustion‖, SAE 2006-01-3385, 2006 [4] Kalghatgi, G. Risberg, P., and Ångström H-E, ―Partially pre-mixed auto-ignition of gasoline to attain low smoke and low NOx at high load in a compression ignition engine and comparison with a diesel fuel‖,
SAE 2007-01-006, 2007 [5] Kalghatgi, G., Hildingsson, L. and Johansson, B., ―Low NOx and Low Smoke Operation of a Diesel Engine using Gasoline-like Fuels ―, ASME Paper # ICES2009-76034
There has been much investment made in developing the engine and after-treatment technology using conventional diesel fuel. Hence, in the short term (< 10 years), conventional diesel fuel (CN > 40) will continue to be used in CI engines. The practical debate on fuel auto-ignition quality in CI engines is about whether future engines will require higher cetane in the 40 to 60 range, not