Engine performance and emissions from a fumigated hydrogen/ammonia compression ignition engine with a hydrogen peroxide pilot

The study investigates, numerically, the potential use of introducing aqueous HO as an ignition promoter in a statistically homogeneous NH/H fuelled, medium speed (1250 rpm), 4-stroke, 1.3 litre cylinder displacement, mildly boosted CI engine with a compression ratio of 17.6:1. The H is considered to be produced on-board from ammonia cracking. An extensive campaign is undertaken using the commercial stochastic reactor model, SRM Engine Suite, which allowed the modelling of temporal, temperature and spatial stratification in the cylinder. The engine performance, combustion phasing, maximum pressure rise rate and emissions (NOx, NO and unreacted NH) are investigated in view of: (i) the share of molecular hydrogen in the initial NH/H mixture from 10 to 40 percent; (ii) the mass of aqueous HO introduced from 0.1 to 16 mg; (iii) the start of injection (−10 to ＋6 CAD aTDC) and duration of injection (1, 4 and 8 CAD); (iv) the amount of exhaust gas recirculation (up to 30 percent by mass); (v) the share of energy from the HO in the aqueous solution mixture at less than 0.5 percent of that in the main fuel; (vi) engine load corresponding to a variation in the equivalence ratio from 0.32 to 1.2 by changing the mass of the NH/H mixture in the combustion chamber. A wide range of loads (evaluated against the engine’s rated power when operated with diesel and at its rated boost levels) can be achieved (44%–93%) with the energy share of HO being as little as equivalent to 2.7% vol% that of the main fuel, ammonia, which is introduced into the cylinder. This implies that the required storage volume of the HO is low, at a few percent that of the main ammonia tank. NOx emissions peak between .6−0.65 and rapidly decrease as the equivalence ratio increases or decreases reaching values marginally above the Tier III standard at high loads (90%) while ammonia slip and NO emissions are generally extremely low (10−12 mg for NH and 0.01 mg/kWh for NO)

Evaluation of Homogeneous Charge Compression Ignition (HCCI) autoignition development through chemiluminescence imaging and Proper Orthogonal Decomposition

Homogeneous Charge Compression Ignition (HCCI) engines deliver high thermal efficiency and, therefore, low CO2 emissions, combined with low NOX and particulate emissions. However, HCCI operation is not possible at all conditions due to the inability to control the autoignition process and new understanding is required. A high-swirl low-compression-ratio, optically accessed engine that can produce overall fuel lean, axially stratified charge (richer fuel mixture close to the cylinder head was achieved using port injection against open valve and homogeneous mixture during injection against closed valve timing) was operated in HCCI mode without and with spark-assist mixture ignition. The present study investigates the differences in the HCCI autoignition process and the propagation of the autoignition front with homogeneous mixture or fuel charge stratification, internal Exhaust Gas Recirculation (iEGR) (introduced by utilizing different camshafts) and spark-assisted iEGR lean combustion. In order to visualize the HCCI process, chemiluminescence flame images, phase-locked to a specific crank angle, were acquired. In addition, time-resolved images of the developing autoignition flame front were captured. Proper Orthogonal Decomposition (POD) was applied to the acquired images to investigate the temporal and spatial repeatability of the autoignition front and compare these characteristics to the considered scenarios. The eigenvalues of the POD modes provided quantitative measure of the probability of the corresponding flame structures. The first POD mode showed higher probability of single autoignition sites originating from a particular location (depending on the scenario). However, the contribution from other modes cannot be neglected, which signified multiple locations of the single autoignition and also, multiple sites of self-ignition of the fuel-air mixture. It was found that increasing iEGR resulted in random combustion (multiple autoignition sites and fronts), which, however, became significantly non-random due to addition of spark-assisted ignition. It was identified in the POD analysis of the time-resolved flame images that the presence of inhomogeneity either in the temperature or the mixture fraction distribution increases the probability of random combustion during the very early stages of flame development. Thus, the fluctuations of heat release is higher during this period

Autoignition initiation and development of n-heptane HCCIcombustion assisted by inlet air heating, internal egr or spark discharge: An optical investigation

An optically accessed, single-cylinder engine capable of operating at both spark ignition and Homogeneous Charge Compression Ignition (HCCI) combustion was used to investigate the difference in the initiation and development of HCCI combustion due to charge stratification, internal Exhaust Gas Recirculation (iEGR) or spark discharge. Natural-light images were acquired to visualise the differences in chemiluminescent structure (i.e. reaction structures) at the early and late stages of formation during HCCI combustion in an attempt to find better ways of controlling HCCI combustion at low and high loads. Regardless of charge stratification, the cycle-to-cycle deviation of autoignition from temporal and spatial repeatability was comparatively small. Flame initiation appeared initially at single or spatially adjacent sites and we did not observe the growth of any new, (i.e. "secondary" in time) reacting 'islands' separate from the original sites. However, with increasing rates of iEGR, the autoignition process was a more spatially random phenomenon, with the development of new, in time, reacting 'islands' being more evident. With spark-assisted HCCI combustion, it was evident that HCCI combustion was advanced by more than 35 crank angle degrees due to the spark discharge but that the combustion process did not appear to propagate in the form of a flame front. Flame initiation would appear at a single spatial location - not always in the vicinity of the spark plug - and we did observe the growth of new, in time, reacting 'islands', separate from the original sites

Autoignition initiation and development of n-heptane HCCIcombustion assisted by inlet air heating, internal egr or spark discharge: An optical investigation

An optically accessed, single-cylinder engine capable of operating at both spark ignition and Homogeneous Charge Compression Ignition (HCCI) combustion was used to investigate the difference in the initiation and development of HCCI combustion due to charge stratification, internal Exhaust Gas Recirculation (iEGR) or spark discharge. Natural-light images were acquired to visualise the differences in chemiluminescent structure (i.e. reaction structures) at the early and late stages of formation during HCCI combustion in an attempt to find better ways of controlling HCCI combustion at low and high loads. Regardless of charge stratification, the cycle-to-cycle deviation of autoignition from temporal and spatial repeatability was comparatively small. Flame initiation appeared initially at single or spatially adjacent sites and we did not observe the growth of any new, (i.e. "secondary" in time) reacting 'islands' separate from the original sites. However, with increasing rates of iEGR, the autoignition process was a more spatially random phenomenon, with the development of new, in time, reacting 'islands' being more evident. With spark-assisted HCCI combustion, it was evident that HCCI combustion was advanced by more than 35 crank angle degrees due to the spark discharge but that the combustion process did not appear to propagate in the form of a flame front. Flame initiation would appear at a single spatial location - not always in the vicinity of the spark plug - and we did observe the growth of new, in time, reacting 'islands', separate from the original sites