Studies of DI Diesel Engine Cold Start Combustion in an Optical Engine J.M. Desantes, J.V. Pastor * , J.M García-Oliver, J.G. Ramírez-Hernández CMT-Motores Térmicos Universidad Politécnica de Valencia, Spain A study on ignition and early combustion development has been performed in an optical engine at in- cylinder thermodynamic conditions representative of those occurring during the cold start of a modern DI Diesel engine for passenger cars at an ambient temperature of -20ºC. A general description of the process has been derived from in-cylinder pressure analysis and high speed imaging, and the effect of most relevant engine parameters has been analysed providing guidelines for cold start optimisation. Introduction peak temperature under motored, but respect- Cold start at low temperatures in current D.I. di- ing distance between piston and cylinder head rect engines is a problem which has not been at TDC so that most flow patterns are not se- properly solved yet and becomes particularly criti- verely modified. cal with current trend to reduce engine compres- • Reduction of engine speed to the lowest con- sion ratio [1]. To promote fuel ignition a heating trollable value (250 rpm) so that engine dynam- plug is usually installed in the cylinder head of ics is reproduced as closely as possible. passenger car DI diesel engines. • External supercharging to achieve peak pres- Although it is clear that there are some key fac- sure values of the real engine during cold start. tors whose control lead to a proper cold start • Control of temperature of all engine fluids (wa- process, their individual relevance and relation- ter, oil and fuel) to the minimum value achieva- ships are not clearly understood [2-6]. Thus, efforts ble to avoid moisture condensation problems on on optimisation of the cold start process are mainly engine surfaces which allows long-duration sys- based on a trial-and-error procedure in climatic tematic experiments to be performed. chambers at low ambient temperature, with serious • External operation of the injection system to limitations in terms of measurement reliability dur- allow skip-fire mode to avoid temperature tran- ing such a transient process, low repeatability and sients, to reduce instability problems near the experimental cost [7-9]. operation limits of the injection system itself, to This paper presents a novel approach for the avoid any engine ECU corrections, to provide study of the first injection cycle of a light duty en- full flexibility to change any injection parameter, gine at -20ºC during the starting process, combin- and to suppress the influence of residuals upon ing visualization tests in an optical engine and heat ignition. release analysis for individual cycles. Systematic • Measurement of instantaneous intake and ex- studies, in the borderline between ignition success haust mass flow rate, so that blow-by can be and misfiring are performed so that the relevance measured indirectly. Such a parameter, which of engine parameters on ignition success can be is not usually controlled in optical engines but assessed. On the basis of these studies, a descrip- can be severely influenced by piston-rings tion of the combustion process under cold start wear, is crucial for the proper estimation of conditions has been derived. mass trapped in the cylinder and, consequently, the real bulk gas temperature. Experimental facility and methodology • External control of glow plug power to control Target in-cylinder conditions to be reproduced heat transfer. in the optical engine around top dead center have • Flexibility for orientation between sprays and been estimated from experiments in climatic glow plug. chamber similar to those made by Payri et al [7]. Peak in-cylinder pressure is between 25 and 30 Preliminary tests showed that the ignition bar, and peak in-cylinder temperature is between process under cold-start conditions can be ex- 335 – 350ºC. tremely variable. Thus, the analysis of consecutive A conventional single-cylinder research engine combustion cycles (with 20 motored cycles be- with optical access through the piston bowl, in a tween them) have shown different combustion fully equipped test cell has been modified for the patterns appearing randomly, as shown in figure 1: study of cold start in the following aspects [10]: in some cycles fuel does not ignite, in some others ignition takes place properly and in some others • Reduction of compression ratio and control of ignition occurs too late to be considered as suc- intake air temperature to achieve in-cylinder cessful ignition for cold start. * Corresponding author: jpastor@mot.upv.es Towards Clean Diesel Engines, TCDE2009 Time ASOE: 1992 us 2656 us 3320 us 4980 us 8 DHRL[J/CAD] 1x10 80 Icumul[-] 400 In-cylinder pressure [bar] 1x107 60 300 1x106 40 200 9960 us 20 100 1x105 0 0 1x104 -5 0 5 10 15 20 25 30 Crank angle [CAD] Fig. 2. Image sequence of the pilot flame combustion with cycle-resolved evolution of pressure and luminosity. Fig.1. Combustion patterns for individual cycles for a single test. Since energy required for fuel evaporation stems not exclusively from the heating plug but Thus, an analysis methodology was defined for also from the surrounding air, this provokes a de- this particular kind of studies combining different crease of local gas temperature which could cause sources of information: flame quenching and slow down or prevent proper • Conventional heat release analysis derived flame propagation. from in cylinder pressure traces, but applied to So, the rest of the fuel of this and the other individual cycles and adapted in some few as- sprays further away from the glow plug will be pects, burnt later, only if a second injection (main) is per- • Light radiation registered with photodiodes and formed. photomultipliers using or not interference filters If main injection is introduced into the chamber to track different radicals, in the period where pilot mass autoignition occurs, • High speed imaging with a CMOS camera at ignition delay for main injection is within the injec- 6000 frames per second under different optical tion event duration. A very steep heat release is configurations and obtained, usually with a single peak. Images have • Other complementary sources of information shown that the intense heat release period corres- such as injection rate and spray momentum ponds to the time interval where a reaction front is flux measurements or outputs from modeling. travelling throughout the combustion chamber (see OH images of fig. 4). This front burns the prepared Results mixture. After the peak, injection is usually over. In order to improve knowledge about the Due to the higher temperature, liquid fuel from the process, only pilot injection test cases (figure 2) piston wall can evaporate and burn and a residual have been considered in addition to single+main combustion process can be observed during the injection cases (figure 3). A general description of rest of the combustion process. In spite of the low the combustion process has been derived, for intensity of this late combustion period, the amount conditions in which combustion succeeds: of energy may reach around 20-30% of the total The injected fuel mixes with air, but due to the energy release. However, this phase is really diffi- low air and engine temperatures evaporation be- cult to control. fore the start of combustion is really poor. Accord- ing to calculations, only around 10-20% of the in- jected mass is evaporated, and most of the evapo- ration proceeds during the injection event. The rest of the injected mass is deposited on the piston surface in liquid state. If heat release starts, some additional evaporation occurs due to combustion- induced heating. Limitations in evaporation are one of the main hurdles for obtaining an acceptable combustion process. After pilot mass is evaporated, most of it under- goes a long autoignition process. Fuel injected close to the glow plug starts burning around 3 ms after the start of injection. However, due to the low Fig. 3. - Image sequence of the pilot+main flame amount of fuel vapour, reaction does not propa- combustion with cycle-resolved evolution of pressure, gate to the rest of the chamber, and heat release is luminosity and rate of heat release. too low to be detected. Injection pressure plays a major role on the ig- nition success and combustion stability. Results show that low injection pressures improve largely ignition probability, since flow velocities are low and consequently, energy dissipation rates. How- ever, if combustion progresses, higher injection pressures lead to better mixing and higher imep is obtained. Other factors analyzed (e.g. injector-glow plug orientation and distance, glow plug temperature, in-cylinder pressure and temperature, or swirl in- tensity) can modify or modulate the picture de- scribed above on the combustion process. Howev- er, further research is necessary to clarify their influences Acknowledgement The authors acknowledge that part of this work has been performed in the frame of the OPTICOMB project (TRA2007-67961-C03-01) funded by the Spanish Ministry of Education and Science. References [1] P. Pacaud, H. Perrin, and O. Laget. Cold Start on Diesel Engine: Is Low Compression Ratio Compati- ble with Cold Start Requirements? SAE Paper 2008- 01-1310 (2008) [2] M.C. Lai, N.A. Henein, X. Xie, T.H. Chue, Y. Itoh and W. Bryzik. Diesel Cold Starting Study Using Optically Accessible Engines. SAE Paper 952366 (1995) [3] Z. Hang Z, N.A. Henein and W. Bryzik. A New Igni- Figure 4. Simultaneous OH (left) & natural luminosity tion Delay Formulation Applied to Predict Misfiring images (right) in a single cycle. During Cold Starting of Diesel Engines. SAE Paper 2000-01-1184 (2000) [4] H. Liu H, N.A. Henein and W. Bryzik. Simulation of A wide range of geometrical and operating pa- Diesel Engines Cold Start. SAE Paper 2003-01-0080 rameters have been considered to identify and (2003) analyse the factors with influence upon cold start [5] I. Osuka, M. Nishimura, Y. Tanaka and M. Miyaki. success in the case of using a conventional plug. Benefits of New Fuel Injection System Technology The election of an adequate injection strategy on Cold Startability of Diesel Engines - Improvement (together with a proper selection of the angle be- of Cold Startability and White Smoke Reduction by tween the spray and the plug) appears as a critical means of Multi Injection with Common Rail Fuel Sys- factor for proper cold start: pilot injection timing tem (ECD-U2). SAE Paper 940586 (1994) [6] N.S. Ayoub and R.D. Reitz Multidimensional Model- and quantity must be such that ignition occurs in ing of Fuel Composition Effects on Combustion and the vicinity of the heating plug. Then, if the delay Cold Starting in Diesel Engines SAE Paper 952425 between the pilot and main injection pulses and (1995) the amount of fuel in the main injection are not [7] F. Payri, A. Broatch, J.R. Serrano, F.L. Rodríguez chosen conveniently, flame quenching can occur and A. Esmoris. Study of the Potential of Intake Air and combustion will not progress. Heating in Automotive DI Diesel Engines. SAE Paper Pilot injection mass seems to be influential in 2006-01-1233 (2006) the amount of heat release generated by the pilot [8] N.A. Henein, A.R. Zahdeh, M.J. Yassine and W. combustion. However, heat required to vaporize Bryzik. Diesel Engine Cold Starting: Combustion In- stability. SAE Paper 920005 (1992) pilot fuel may also have a large influence on this [9] K. Mitchell. The Cold Performance of Diesel Engines. combustion process. SAE Paper 932768 (1993) Main injection fuel quantities also play a role. [10] J.V. Pastor, J.M. Garcia-Oliver, J.M. Pastor and J.G. Due to the low evaporation rate, larger main injec- Ramirez-Hernandez. Experimental facility and me- tion masses mean a larger amount of vapour fuel, thodology for Cold Start studies in Diesel engines. and thus a higher heat release, once the ignition Submitted to Meas. Sci. Technol. (2009) phase has been overcome.