Multiple Premixed Compression Ignition Mode Research Articles

Optimization of piston bowl and valve system in compression ignition engine fueled with gasoline/diesel/polyoxymethylene dimethyl ethers for high efficiency

Polyoxymethylene dimethyl ethers, with excellent volatility and oxygen content of up to 49%, have great potential to improve engine performance and emission characteristics. In this study, experiments were carried out in a single-cylinder engine fueled with gasoline/diesel/polyoxymethylene dimethyl ethers blend fuel using multiple premixed compression ignition combustion mode along with engine optimization to exploit the high-efficiency potential. The thermal efficiency was increased by 9.4 percentage points after transforming the combustion mode from conventional diesel mode to gasoline/diesel/polyoxymethylene dimethyl ethers multiple premixed compression ignition mode. A fully variable valve system and a redesigned low-heat-transfer piston were used to further improve the thermal efficiency. The low-heat-transfer piston had a 15% lower area–volume ratio compared with the original ω-type piston. By replacing the original ω-type piston with the low-heat-transfer piston, the heat transfer loss was reduced by 2.29 percentage points and thus indicated thermal efficiency could be increased by 2.37 percentage points, which was up to 50.03%. On the basis of the low-heat-transfer piston, indicated thermal efficiency could be further increased to 51.09% with the application of fully variable valve system due to the longer ignition delay and more premixed combustion. At the same time, NOX emissions could be controlled below 0.4 g/kW·h using high exhaust gas recirculation ratio, which equaled the NOX emission limit of Euro VI standard. Although soot emissions could be increased due to weak turbulence and insufficient intake charge using the low-heat-transfer piston and fully variable valve system, it was still lower than those of the original diesel engines.

An experimental investigation on thermal efficiency of a compression ignition engine fueled with five gasoline-like fuels

Partially Premixed Compression Ignition (PPCI) and Multiple Premixed Compression Ignition (MPCI) are two high efficiency and clean combustion strategies. This paper presents an experimental investigation on the efficiency of PPCI and MPCI combustion using five gasoline-like fuels: gasoline, gasoline/ethylhexyl-nitrate (EHN) blends, gasoline/diesel blends, gasoline/polyoxymethylene dimethyl ethers (PODEn) blends, gasoline/diesel/PODEn blends, denoted as G, GE, GD, GP, GDP, respectively. The study was conducted using a single cylinder engine that was retrofitted from a production Euro IV diesel engine. The results showed that in PPCI mode, the order of thermal efficiency from the highest to the lowest was GP, G, GDP, GE, and GD, while the order in MPCI mode was GP, GDP, GE, G, and GD. The thermal efficiency of MPCI mode was higher than that of PPCI mode, except for GP, whose thermal efficiency was the highest of about 48.8% for both modes due to the early CA50 and short combustion duration. In PPCI mode, NOx and soot emissions were below 0.4g/kWh and 0.01g/kWh, respectively, satisfying the Euro VI emission standard. NOx emission in MPCI mode can meet the requirement of Euro VI. However, the soot emission exceeded the limit because the second stage combustion was partially diffusion combustion, implying an extra Gasoline Particle Filter (GPF) was needed to meet the Euro VI standard. This study indicates that PPCI injection strategy alone and MPCI injection strategy with GPF are two potential ways of retrofitting a Euro IV engine to meet the Euro VI emission standard. GPF increases manufacturing cost, but higher thermal efficiency in MPCI mode could reduce the total cost of ownership due to the lower fuel consumption.

Exploiting new combustion regime using multiple premixed compression ignition (MPCI) fueled with gasoline/diesel/PODE (GDP)

GDP is a blend fuel containing gasoline, diesel, and polyoxymethylene dimethyl ethers (PODE). Previous study has shown that by combining premixed ratio and high fuel oxygen content, GDP partially premixed combustion ignition (PPCI) broke the trade-off between NOx and soot with moderate exhaust gas recirculation (EGR) while maintaining high engine efficiency. In this work, multiple premixed compression ignition (MPCI) mode was adopted to break the trade-off between fuel economy and combustion noise, which is the major challenge of the single injection GDP PPCI combustion. In this work, the MPCI adopted a double injection strategy with the second injection timing fixed at 6 crank angle degree (CAD) before top dead center (BTDC). The effects of first injection timing and split injection ratio on emissions and combustion were investigated. The results showed that GDP MPCI mode exhibited two-stage combustion. With the advance of first injection timing and the decrease of first injection quantity, the amount and rate of the first-stage heat release decreased, and those of the second-stage heat release increased. Compared with single injection PPCI, MPCI reduced the maximum HC and CO emissions by 59% and 44%, respectively. Soot emissions of MPCI mode were below 0.01g/kW.h, while NOx emissions were controlled under 0.4g/kW.h. The results indicated that MPCI mode could improve engine efficiency and reduce combustion noise simultaneously (low MPRR), and the optimal engine performance and emissions were achieved with 26–31 CAD BTDC first injection timing and 30–40% first injection ratio.

Temporally and spatially distributed combustion in low-octane gasoline multiple premixed compression ignition mode

An advanced combustion strategy namely “multiple premixed compression ignition” (MPCI) can exploit multiple combustion regimes. In this paper, the two-stage premixed combustion (i.e. low-temperature combustion and high-temperature combustion) using relatively low-reactivity and high volatility fuel is uncovered in MPCI mode. The mixture formation and combustion processes of low-octane gasoline MPCI mode is resolved by KIVA–CHEMKIN. The low-octane gasoline (G70H30) modeled in this study is a mixture of 70% gasoline and 30% n-heptane by volume, and it is represented by iso-octane and n-heptane oxidation mechanism. A discrete multi-component (DMC) vaporization model is used to simulate the evaporation process of G70H30, while a simplified 12-step GRI-Mech model and a two-step soot model are applied for the NOx and soot calculation, respectively. As a comparative study, partially premixed compression ignition (PPCI) mode of G70H30 is also simulated using the same numerical models. From the longitudinal and transverse sections of the combustion chamber, temporally and spatially distributed combustion events are found in the MPCI mode. That is to say, the first low temperature premixed compression ignition occurs outside of the piston bowl, and it occurs mainly in the squeeze zone above the piston triggered by the local fuel rich region. Then the second high temperature premixed combustion, which is controlled by the local turbulence resulting from the second fuel injection and chamber wall interaction, is located in the center of the combustion chamber. The temporally and spatially distributed combustion in MPCI mode decreases the local temperature, resulting in less NOx formation and lower heat transfer losses. However, there is no marked combustion distribution phenomenon in the G70H30 PPCI mode, and more NOx is generated due to higher local temperature. In addition, the early first injection of MPCI and PPCI causes spray-liner impingement, and becomes the main source of soot formation in the subsequent combustion processes.

Combustion and emission characteristics of Multiple Premixed Compression Ignition (MPCI) fuelled with naphtha and gasoline in wide load range

This paper investigates the effect of naphtha (RON=65.6) and commercial gasoline (RON=94.0) on Multiple Premixed Compression Ignition (MPCI) mode. The experiment is conducted on a single cylinder research diesel engine with compression ratio of 16.7. The engine is operated at an engine speed of 1600rpm for the IMEP from 0.4 to 1.4MPa. Commercial diesel (CN=56.5) is also tested in Conventional Diesel Combustion (CDC) mode as a baseline. At each operating point, the injection strategy and intake conditions are adjusted to meet with the criteria (NOx<0.4g/kWh, soot<0.06m−1, MPRR<1MPa/deg and CA50<20 CAD ATDC). The typical two-stage combustion characteristics of MPCI are obtained in both naphtha and gasoline. Stable combustion is achieved by naphtha in wide load range, while the engine fuelled with gasoline cannot operate stably at 0.4MPa IMEP. The COV of IMEP of gasoline MPCI is higher than that of naphtha and diesel. However, gasoline has the low MPRR and the retarded CA50 at medium and high loads due to its longest ignition delay. As a result of low exhaust loss for naphtha and low heat transfer loss for gasoline, the thermal efficiencies are higher for both naphtha and gasoline in MPCI mode than diesel in CDC mode, even though diesel has the highest combustion efficiency. The separated combustion in MPCI leads to low cylinder temperature, and moderate EGR ratio (less than 30%) is needed to control NOx emissions under the limit of EURO VI.

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.

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.

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.

Gasoline multiple premixed compression ignition (MPCI): Controllable, high efficiency and clean combustion mode in direct injection engines

A novel combustion concept namely “multiple premixed compression ignition” (MPCI) in gasoline direct injection compression ignition (GDICI) regime is proposed. Its predominant feature is the first premixed and followed quasipremixed combustion processes in a sequence of “spray-combustion-spray-combustion”. The multiple-stage premixed combustion decouples the pressure rise with pollutants formation process, which means the pressure rise rate and emissions can be reduced simultaneously, while achieving a high thermal efficiency. The gasoline MPCI mode has been demonstrated in a research engine with a compression ratio of 18.5. Gasoline with the research octane number (RON) of 94.4 was tested under 1400 rpm, 0.6 MPa IMEP conditions, without EGR and intake boosting. A parameter study of common rail pressure and intake temperature was implemented to investigate their effects on the performance of MPCI mode. Compared to the single-stage diffusion combustion in traditional diesel engines, the gasoline MPCI mode achieves lower emissions of soot, NO, CO, as well as slightly higher indicated efficiency, with a penalty of higher THC emissions when the common rail pressure is larger than 80 MPa in this study. With intake temperature sweeping, the gasoline MPCI mode also has the foregoing advantages compared to the diesel under the same operating conditions.

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.

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.