Compression Top Dead Center Research Articles

Numerical investigation of the effects of intake pipe deflection angles on the in-cylinder flow and vortex structures of a cycloidal rotary engine

Intake pipe structure has a pivotal impact on the distribution of vortices within the combustion chamber in a cycloidal rotary engine (CRE). Therefore, studying the influence of the intake pipe deflection angle (IPDA) on the in-cylinder airflow motion has significant importance for enhancing CRE performance. This study utilized computational fluid dynamics and chemical reaction kinetics methods to establish a numerical simulation model for the in-cylinder flow and combustion in the CRE. Subsequently, the Omega vortex identification method was employed to investigate the influence of IPDA on the vortex structures within the cylinder and to explore the relationship between CRE performance and the vortices. The research findings indicate that although the IPDA did not significantly alter the fuel mass injected into the cylinder, it increased the airflow velocity by 14.6% during the main intake stage and increased the mass fraction of the burned fuel at the compression top dead center by 19.1%. Additionally, the increased airflow velocity within the cylinder led to improvements in both the mean tumble ratio by 186.5% and the turbulent kinetic energy by 25.5%. Furthermore, the IPDA significantly changed the distribution of vortices within the cylinder, which is a key factor contributing to the combustion variation of the CRE. The case of IPDA = 16° provided the largest volume of the strong vortices and the highest mean in-cylinder pressure. Compared to the original design, the volume of strong vortices was 1323.6% greater, and the mean in-cylinder pressure was higher by 5.3%.

Study on the Effect of Secondary Injection on the Combustion and Emission of n-Butanol Engines.

n-Butanol, as a biological alternative fuel containing oxygen, has similar physical and chemical properties to gasoline and has a wide range of sources, which has attracted more and more attention and research. Direct injection technology has been widely used in the field of internal combustion engine due to its advantages of flexibility and control ability. In this paper, the secondary injection of n-butanol engine under the mode of in-cylinder direct injection is discussed to organize stratified combustion of the mixture, optimize combustion to improve the thermal efficiency, and reduce emission. A four-cylinder four-stroke spark ignition (SI) engine was selected to carry out the secondary injection experiment of n-butanol under the excess air ratio (λ) of 1, an engine speed of 1500 r/min, and a low load, and the variables were the second injection ratio and timing. The results show that the secondary injection of n-butanol can achieve stratified combustion of the mixture, but only at a specific second injection timing such as 100°CA before compression top dead center (BTDC) or 125°CA BTDC, the combustion effect is the best. A small second injection ratio can optimize combustion, improve brake thermal efficiency, and reduce hydrocarbon and carbon monoxide emissions. When the second injection ratio is greater than 60%, it results in incomplete fuel combustion, a 3 to 4% reduction in thermal efficiency, and an increase in emissions. Coefficient of variation (COV) was increased by secondary injection, but the effect was insignificant in the small injection ratio, and it will increase with the increase of the second injection ratio. The change of particle number is mainly affected by the nuclear particle number, and with the increase of the second injection ratio, the total particulate number is more affected by the second injection timing. The second injection ratio of 40% can reduce the total particle number under the mixed-gas stratification condition.

Experimental characterization of the turbulent intake jet in an engine flow bench

The turbulent intake flow of an optically accessible internal combustion engine is modeled using an air flow bench to reduce the complexity in the number of variables inherent within engine flows. By removing the piston and introducing a new optically accessible housing and outlet channel, the flow bench design simulates engine flows in the region just downstream of the intake valves and offers the possibility to measure and calculate quantities that would be difficult or impossible to obtain in the unsteady environment of a dynamic engine. Velocity data obtained via high-speed particle image velocimetry of the flow bench in the symmetry and valve planes are compared with data from a base operating condition of the motored engine at an intake pressure of 0.95 bar and a speed of 800 rpm at - 270°CA (270 crank-angle-degrees before compression top dead center), beginning with stationary valves at the corresponding valve lift of - 270°CA, then with moving valves. Analysis of the intake jet turbulence for increasing mass flow rates reveals a coherent flapping of the jet at a frequency of 752.5 Hz for only the 100% mass flow rate case. The vortex shedding frequency of the valve stem is estimated to being in the range of 634–799 Hz, indicating a possible link between the coherent jet flapping and the vortex shedding surviving the acceleration through the valve gap. Through comprehensive analysis, this study provides valuable validation data and insight into the intake flows of internal combustion engines.

Effect of spark timing on the pre-chamber jet ignition in a lean-burn natural gas engine

An enhanced combustion concept, pre-chamber jet ignition can provide high ignition energy and a wide dispersion of ignition sources, thus improving combustion performance and realizing stable lean combustion. However, the ignition performance of the pre-chamber jets depends strongly on the pre-chamber combustion process and is sensitive to many factors. Therefore, this paper will explore the impact of spark timing on the pre-chamber combustion, turbulent jet characteristics, and main chamber combustion performance by CFD simulation based on an active pre-chamber natural gas engine. The in-cylinder flow and combustion processes of the engine are calculated with the fuel-air equivalence ratio of the main chamber maintained at 0.5, and the spark timing varied from 684°CA to 708°CA with a step of 3°CA (compression top dead center is 720°CA). The results show that, in the present engine operating point, the pre-chamber combustion heat release rate is improved when the spark timing is retarded from 696°CA to 705°CA, and the injection velocity of the pre-chamber jet accordingly increases. As the spark timing continues to be delayed until 708°CA, the pre-chamber combustion deteriorates, and the injection velocity of the pre-chamber jet decreases. Under the combustion phase CA50 no more than 725°CA and the combustion duration CA5-90 fixed around 30°CA, the IMEP will maintain a relatively stable value when the spark timing is delayed from 693°CA to 705°CA, in the meantime, the NOx emission decreases with the delay of spark timing. Therefore, on the premise of ensuring engine power performance, it is a feasible scheme to delay spark timing to reduce NOx emission for the natural gas engine with an active pre-chamber.

Effect of internal exhaust gas recirculation on performance, combustion, and emissions in a PFI camless engine

AbstractThe internal exhaust gas recirculation (EGR) into the intake manifold has a significant impact on fuel economy and emissions in the spark ignition (SI) engine. In this study, an in‐house developed novel electro‐pneumatic variable valve actuation (VVA) system was integrated on a PFI conventional engine and modified into PFI camless engine to vary the valve events independently. The potential of the developed VVA system in achieving the internal EGR by varying the intake valve opening (IVO) timings was widely investigated. The IVO timing was varied from 30° CA (crank angle) bITDC (before intake top dead center) to 40° CA aITDC (after intake top dead center). The spark timing was varied from 10° to 30° CA bCTDC (before compression top dead center). A 41% pumping mean effective pressure reduction was achieved with varying the IVO from 40° to −30° CA aITDC. It is due to the internal exhaust gas recirculation into the intake manifold that reduces the differential pressure between the manifold and combustion chamber. The entrapment of exhaust gas in the intake manifold dilutes the fresh charge, which deteriorates the combustion. However, advancing the spark timing from 10° to 30° CA bCTDC at IVO −30° CA aITDC, an 18% gas excahnge efficiency improvement was obtained. The fresh charge dilution forms an inhomogeneous mixture inside the cylinder that results in lower NOx emission. It is concluded that the developed VVA system has the potential to improve part loads efficiency by utilizing the internal EGR in a PFI camless engine.

Analysis of Hydrocarbon Mixture Performance of a Dual-Channel Swirl Engine

In order to understand and improve the oil and gas mixing performance of a dual-channel vortex chamber diesel engine, the BH175F dual-channel vortex chamber combustion system was used as the research foundation, and the oil-gas mixture process of the combustion system was numerically analyzed. By analyzing the cylinder temperature, cylinder pressure, mixing process and combustion process of the combustion system, the mixture performance of the combustion system was studied. Results indicated that: The mixture of the compression Top Dead Center (TDC) started to enter the main combustion chamber through the start-up hole; when the piston reached 4° After Top Dead Center (ATDC), the mixture started to enter the main combustion chamber through the connecting channels A and B, and the high-concentration mixture entered the main combustion from the start-up hole; when the piston continued running down to 20° and 25° ATDC, it could be seen that the main combustion chamber mixture was already relatively uniform; besides, when the equivalence ratio was between 0.8 and 1, the air-fuel mixture was unevenly distributed in the main combustion chamber. It provides guidance for further improvement of the combustion system.

Experimental Study of Combustion Fluctuation Reduction Using In-Cylinder Pressure Estimation in Gasoline Engine

Gasoline engines needs to reduce its negative emission waste and raise its thermal efficiency. Previous studies have shown an improvement of engines by regulating the ignition timing and retaining the engine at certain air-to-fuel ratio. Additional development of the thermal efficiency is anticipated by reducing the oscillation of pressure due to combustion (referred to as combustion fluctuation) during each cycle. Reducing the combustion fluctuations promotes the generation of a stable combustion field and improves fuel consumption. Since the combustion fluctuations are significantly affected by the in-cylinder pressure at compression top dead center (referred to as TDC pressure), the present study proposes a method to estimate the TDC pressure in the next cycle. The estimation was conducted by measuring the in-cylinder pressure at exhaust valve opening in the given cycle. This study also developed the method to reduce the combustion fluctuations by using the TDC pressure estimation and controlling the ignition timing. In our experiments, it was found that the developed methods reduced the fluctuations of the indicated mean effective pressure (IMEP), the maximum in-cylinder pressure, and the TDC pressure by 62.1%, 51.2%, and 38.5%, respectively.

Numerical and experimental investigations of the effects of the second injection timing and alcohol-gasoline fuel blends on combustion and emissions of an HCCI-DI engine

In this study, experimental and numerical investigations of a homogeneous charge compression ignition (HCCI) combustion engine were performed using alcohol-gasoline fuel blends and two-stage direct injection (TSDI) strategy. A diesel engine was modified to operate as an electronically controlled HCCI-DI engine. TSDI strategy was applied by fixing the first injection timing in intake stroke and varying the second injection timing close to the compression top dead center (TDC). The selected fuels were pure gasoline and four different blends of ethanol and methanol with gasoline, namely E10, E20, M10 and M20. The effects of the second injection timing and alcohol-gasoline fuel blends on the HCCI combustion and emissions were investigated at constant engine speed and high equivalence ratio for the same energy input. CFD simulations were performed using AVL Fire code and CFD results were compared with the experimental results of the HCCI-DI engine. Cylinder gas pressure, rate of heat release (ROHR), maximum cylinder gas temperature, CO and NOx emissions were investigated numerically. Visual information about the in-cylinder temperature distribution and NOx emissions were provided from images taken from the CFD model. It can be understood from both experimental and CFD studies that combustion phase can be most effectively controlled by changing the second fuel injection timing for all fuel blends. Adding alcohol to gasoline helped to decrease NOx emissions while keeping the maximum cylinder gas pressure stable compared to pure gasoline.

Study on Optimization of Control Parameters for the Second Cycle during Forward Rotation Direct-Start Process at a GDI Engine

To get better starting performance for GDI engine forward rotation direct-starting without starter, the optimized choices of fuel injection and ignition control parameters under different initial start conditions was carried out in a wall-guided GDI engine. The results showed that the ignition timing and injection timing have been less affected by the initial start conditions. When the ignition timing is kept near its compression TDC and the injection timing is kept within the scope of 2-10 °CA after the crankshaft rotation, the engine all can get higher rotation speed under different initial start conditions. When the fuel pressures and cooling water temperature are lower, the richer mixture was needed for obtaining higher rotation speed.

Load expansion of naphtha multiple premixed compression ignition (MPCI) and comparison with partially premixed compression ignition (PPCI) and conventional diesel combustion (CDC)

In previous studies, multiple premixed compression ignition (MPCI) has been proposed as a novel combustion concept in gasoline compression ignition engines which has great potential to achieve high thermal efficiency and low emissions simultaneously. MPCI mode was realized by a sequence of “spray–combustion–spray–combustion” around the compression top dead center (TDC). This study is aimed for the high load expansion of naphtha MPCI. In addition, the study investigated advantages and disadvantages of MPCI compared with partially premixed compression ignition (PPCI) and conventional diesel combustion (CDC). Engine operating range successfully reached indicated mean effective pressure (IMEP) of 1.4MPa with high thermal efficiency, low emissions and acceptable combustion noise by the optimization of the injection parameters and the intake management.For MPCI, earlier combustion phasing was possible even at the high load operation compared with PPCI and CDC. This was attributed to the separated heat release characteristics and pressure rise rate process. The divided pressure rise rate process caused considerably low maximum pressure rise rate (MPRR) characteristics such as 0.8MPa/deg at IMEP 1.4MPa condition. The earlier combustion phasing led to the higher thermal efficiency characteristics of MPCI combustion compared with PPCI and CDC. This was attributed to the lower exhaust heat loss characteristics. However, high level of hydrocarbon (HC) and carbon monoxide (CO) emissions with low combustion stability at the low load operation were considered as severe challenges to overcome.

Endoscopic temperature imaging in a four-cylinder IC engine via two-color toluene fluorescence

Building on the development of a large-aperture, UV-transparent endoscope designed specifically for use in IC engines, the gas-phase temperature in a fired, multi-cylinder engine was imaged based on laser-induced fluorescence (LIF) of a fuel tracer. Laser light at 266nm was formed into a light sheet via a laser-input endoscope and excited fluorescence of toluene, port-fuel injected in a mixture with the base fuel iso-octane. The resulting UV-LIF signal was collected by an endoscope head in the combustion chamber, split into two wavelength-channels by a dichroic beam splitter, and detected on two separate cameras. Exploiting the temperature-dependence of the LIF spectrum, quantitative images of temperature were derived from the pixel-wise ratio between the two images. We describe the procedures for cross-registration of the two images and calibration of the LIF-temperature conversion, which are more challenging compared to fully optically-accessible engines. To assess the systematic error we performed a quasi-dimensional simulation matched in detail to engine data. Between intake-valve closure and mid-compression, the temperatures derived from the model and that from LIF agreed within 5K, while the difference increased to 50K at 28CA before compression top-dead center. In addition to such quantitative imaging in the compression stroke, unburnt fuel was detected in the residual gas. This feature unexpectedly enabled qualitative imaging even after combustion. Predicted by the simulation, the initial back-flow of exhaust gas into the intake during gas exchange could be visualized on a single-shot basis.

Effect of Injection Timing on PPCI and MPCI Mode Fueled With Straight-Run Naphtha

Partially premixed compression ignition (PPCI) and multiple premixed compression ignition (MPCI) mode of straight-run naphtha have been investigated under different injection strategies. The MPCI mode is realized by the multiple premixed combustion processes in a sequence of “spray-combustion-spray-combustion” around the compression top dead center. The spray and combustion events are preferred to be completely separated, without any overlap in the temporal sequence in order to ensure the multiple-stage premixed compression ignition. The PPCI mode is well known as the “spray-spray-combustion” sequence, with the start of combustion separated from the end of injection. Straight-run naphtha with a research octane number (RON) of 58.8 is tested in a single cylinder compression ignition engine whose compression ratio is 16.7 and displacement is 0.5 l. Double and triple injection strategies are investigated as the last injection timing sweeping at 1.0 MPa IMEP and 1800 rpm conditions. The MPCI mode is achieved using the double injection strategy, but its soot emission is higher than the PPCI mode under triple injection strategy. This is mainly because of the lower RON of the straight-run naphtha and the ignition delay is too short to form an ideally premixed combustion process after the second injection of straight-run naphtha. Diesel fuel is also tested under the same operating conditions, except for employing a single injection strategy. The naphtha PPCI and MPCI mode both have lower fuel consumption and soot emission than diesel fuel single injection mode, but the THC emissions are both higher than that of diesel fuel.

Performance of straight-run naphtha single- and two-stage combustion modes from low to high load

Double injection strategies with single-stage heat release and two-stage heat release process of straight-run naphtha were investigated on a single-cylinder diesel engine from low to high load. The two-stage combustion strategy is realized by split spray and combustion events around the compression top dead center with a dominant feature of “Combust After Injection End, Inject After Combustion End” to ensure the premixed compression ignition. The single-stage combustion is realized by the “spray–spray–combustion” process with the start of combustion separated from the end of injection. The straight-run naphtha has a research octane number of 58.8, and the compression ratio and displacement of the test engine are 16.7 and 0.5 L. Double injection strategy is used to generate the single- and two-stage combustion modes with different injection timing. NOx and total hydrocarbon emissions of the two-stage combustion mode are lower than that of single heat release mode in this study, and it is much easier to produce two-stage combustion mode at higher engine load. Diesel is also tested under double injection strategy just as the single heat release mode of straight-run naphtha, but the fuel efficiency and emission performance are worse than that of naphtha.

Comparative Study of Low Octane Gasoline Multiple Premixed Compression Ignition and Conventional Diesel Combustion

This article proposes a novel combustion mode, namely “multiple premixed compression ignition” (MPCI), in gasoline direct-injection compression ignition (GDICI) regime. Its predominant feature is the multiple premixed combustion processes in a sequence of “spray–combustion–spray–combustion” around the compression top dead center. The multiple-stage premixed combustion decouples the pressure rise with the pollutant formation process. With optimization, it can lower the rate of pressure rise and emissions simultaneously while achieving high thermal efficiency. The experimental study has been carried out in a single-cylinder research engine with a compression ratio of 18.5. Gasoline with the research octane number of 66 was tested under MPCI mode with different exhaust gas recirculation rates and injection timings. Compared to the single-stage diffusion combustion mode of conventional diesel engines, the low octane gasoline MPCI mode achieves lower emissions of smoke, NO, and CO as well as greater thermal efficiency, with a penalty of greater total hydrocation emissions.

New premixed compression ignition concept for direct injection IC engines fueled with straight-run naphtha

Straight-run naphtha is tested in the newly raised combustion concept, called multiple premixed compression ignition (MPCI) mode, on a single cylinder diesel engine under different speeds. The partially premixed compression ignition (PPCI) mode is also investigated for comparison. The MPCI mode proposed by the authors was realized by multiple premixed combustion processes in a sequence of “spray–combustion–spray–combustion” around the compression top dead center (TDC). The spray and combustion events are preferred to be separated completely, with a dominant feature of “Combust After Injection End, Inject After Combustion End” to ensure the multiple-stage premixed compression ignition. The PPCI mode is well known as the “spray–spray–combustion” sequence with the start of combustion (SOC) separated from the end of injection (EOI). The straight-run naphtha has a research octane number (RON) of 58.8, and the compression ratio and displacement of the test engine is 16.7 and 0.5L. Double injection strategy is employed to generate the PPCI and MPCI modes. Maximum pressure rise rate and NOx emission of the MPCI mode is lower than that of PPCI mode, and it is easier to produce MPCI mode at lower engine speed. Diesel is also tested under double injection strategy just as the PPCI mode, but the fuel efficiency and emission performance is worse than that of straight-run naphtha.

Parameter study of common rail pressure for low octane gasoline multiple premixed compression ignition (MPCI) in light-duty diesel engine

A novel combustion concept called ‘multiple premixed compression ignition’ in the gasoline direct injection compression ignition regime is proposed. Its predominant feature is the first premixed and subsequent quasi-premixed combustion processes in a sequence of ‘spray–combustion–spray-combustion’ around the compression top dead center. The multiple-stage premixed combustion decouples the pressure rise from the pollutants formation process, which means the pressure rise rate and emissions can be reduced simultaneously, while achieving a high thermal efficiency. The experimental study of the multiple premixed compression ignition mode has been carried out in a single cylinder research engine retrofitted from a light-duty diesel engine with a compression ratio of 18.5. Low octane gasoline with the research octane number of 66 was tested under 1400 r/min, 0.8 MPa indicated mean effective pressure conditions, without exhaust gas recirculation or intake conditioning. A parameter study of common rail pressure was implemented to investigate its effect on the performance of the multiple premixed compression ignition mode. When the common rail pressure is 60 MPa under the foregoing operating conditions, the shortest ignition delay of the test fuel is observed. In addition, compared to the single-stage diffusion combustion in traditional diesel engines, the multiple premixed compression ignition mode with the test fuel achieves lower emissions of soot, NO and CO, as well as slightly higher indicated efficiency, with a penalty of higher total hydrocarbon emissions.

Friction Characteristics of Steel Pistons for Diesel Engines

The use of iron pistons is increasing due to the higher power requirements of diesel truck engines. Expansion of the iron piston is a common concern. The purpose of this study is to clarify the lubrication conditions of cast iron and steel pistons. Friction of steel and cast iron pistons were measured using the floating liner method. The friction of conventional aluminum pistons was also measured for comparison in this study. The measurement of the secondary motion of each piston was carried out to analyse piston friction, and the following findings were obtained. The cast iron piston showed boundary lubrication at compression top dead center (TDC). This is presumably caused by the larger tilting angle of the piston induced by the larger piston clearance. Steel pistons showed hydrodynamic lubrication conditions at TDC and BDC through each cycle, due to a good oil supply.

Fuel octane effects on gasoline multiple premixed compression ignition (MPCI) mode

This paper proposes a novel combustion concept namely “multiple premixed compression ignition” (MPCI) in gasoline direct injection compression ignition (GDICI) regime. Its predominant feature is the first premixed and followed quasi-premixed combustion processes in a sequence of “spray-combustion-spray-combustion” around the compression top dead center. The multiple stage premixed combustion decouples the pressure rise with pollutants formation process. With the optimization of operating parameters such as injection timing and split ratio, it can lower the pressure rise rate and emissions simultaneously while achieving a high thermal efficiency at high engine load. The experimental study of the gasoline MPCI mode has been carried out in a single cylinder research engine retrofitted from a light-duty diesel engine with a compression ratio of 18.5. Gasoline with the research octane number (RON) of 66, 76 and 86 were tested under 1400rpm, 0.8MPa IMEP conditions as injection timing sweeping, without EGR and intake conditioning. Compared to the single-stage diffusion combustion mode of conventional diesel engines, the RON66 MPCI mode achieves lower emissions of soot, NO, CO, as well as higher thermal efficiency, with a penalty of higher THC emissions. For RON76 and RON86, it is harder to realize the same performance due to a poorer auto-ignition quality than RON66.

A Study of the Effects of Biofuel Use on Piston Lubrication During Fuel Post Injection in a Direct Injection Diesel Engine

The reduction of both exhaust gases and carbon dioxide emissions is necessary to meet future emissions regulations for diesel engines. Exhaust after-treatment devices are gradually being applied to diesel engines to reduce exhaust gases. Diesel particulate filters (DPF), an after-treatment device for diesel engines, in some cases require fuel post injection for regeneration. Post injection is usually conducted at the midpoint of the expansion stroke, and therefore causes fuel adhesion to the cylinder wall. However, using biofuels in a diesel engine is an effective way of reducing carbon dioxide emissions. It is well known that biofuels are chemically unstable, but the effects of biofuels on piston lubrication condition have not been thoroughly studied. In this study, piston lubrication condition during post injection in a single cylinder DI diesel engine using biofuel was investigated. Piston and ring friction forces were measured under engine operating conditions by means of a floating liner device to investigate the lubrication condition of the piston and rings. Both light fuel oil and biofuel were used in the measurements, with rapeseed methyl ester (RME) being used as the biofuel. Lubricating oil on the cylinder wall was also sampled under engine operating conditions, and the effect of post injection on fuel adhesion to the cylinder wall was analyzed. It was found that the effect of post injection on fuel adhesion to the cylinder wall was remarkable around the top dead center (TDC), and the fuel dilution rate reached approximately 90%. The results of the measurement of the piston friction forces showed that post injection caused an increase in the friction forces at the compression TDC (CTDC) in the cases of both RME and light fuel oil, and the friction forces at CTDC increased according to the delay of the post injection timing. The increase in the piston friction forces was moderate in the case of RME. It seems that the higher viscosity and the oiliness of RME suppressed the increase in piston friction forces at TDC. The following effects were found in this study. Fuel post injection caused fuel adhesion to the cylinder wall. Such phenomena affected the lubrication condition of the piston. In the case of RME, the increase in the piston friction forces caused by post injection was smaller than that of light fuel oil, but the effects on piston lubrication condition in the case of using other biofuels needs to be investigated.

Influences of pilot injection and exhaust gas recirculation (EGR) on combustion and emissions in a HCCI-DI combustion engine

The HCCI-DI combustion mode was achieved in a heavy-duty diesel engine using the early pilot injection in the intake stroke and the main injection around compression top dead center (TDC). The effects of pilot injection quantity and EGR rate on HCCI-DI combustion and emissions were investigated. NOx emission has a dramatically decrease as the pilot injection quantity increases, but it is still in a high level that needs to be reduced. The smoke emission has a slight increase with the rise of pilot quantity, but it is always in a low level. The increasing HC and CO emissions can be easily removed by the diesel oxidation catalyst (DOC). The HCCI-DI combustion with low level of EGR is an effective method to reduce NOx emission further, and smoke emission increases as EGR rate increases, but it is in an acceptable level. The HCCI-DI combustion mode operating range is limited by the peak of cylinder pressure, the pressure rise rate and the cycle-to-cycle variations. There are optimal EGR rates and pilot quantities to achieve low NOx and low smoke emissions.