Isobaric Combustion at a Low Compression Ratio

In a previous study, it was shown that isobaric combustion cycle, achieved by multiple injection strategy, is more favorable than conventional diesel cycle for the double compression expansion engine (DCEE) concept. In spite of lower effective expansion ratio, the indicated efficiencies of isobaric cycles were approximately equal to those of a conventional diesel cycle. Isobaric cycles had lower heat transfer losses and higher exhaust losses which are advantageous for DCEE since additional exhaust energy can be converted into useful work in the expander. In this study, the performance of low-pressure isobaric combustion (IsoL) and high-pressure isobaric combustion (IsoH) in terms of gross indicated efficiency, energy flow distribution and engine-out emissions is compared to the conventional diesel combustion (CDC) but at a relatively lower compression ratio of 11.5. The experiments are conducted in a Volvo D13C500 single-cylinder heavy-duty engine using standard EU diesel fuel. The current study consists of two sets of experiments. In the first set, the effect of exhaust gas recirculation (EGR) is studied at different combustion modes using the same air-fuel ratio obtained from the preceding work. In the second set of experiments, different injection strategies are investigated for IsoL and IsoH combustion at constant and varying load conditions. From the results, it is found that isobaric combustion has similar or higher gross indicated efficiency than those of CDC. The exhaust losses are higher while the heat transfer losses are lower than CDC, which could be beneficial for DCEE concept. For isobaric cases, the NOx emissions were lower with higher uHC/CO/Soot emissions compared to CDC. From the injection strategy study, it was found that the gross indicated efficiency is highest with three injections i.e. at medium load. The efficiency is lower for both low and high load conditions due to increased exhaust and heat transfer losses, respectively. Also, the gross indicated efficiency is largely unchanged when more than one injection event is executed; however the IsoL yields higher overall emissions as compared to IsoH combustion. Introduction Compression ignition engines operated with conventional diesel fuel are widely used in stationary and transportation energy sectors due to high engine efficiency and reliability advantages. However, attaining the strict emission regulations of nitrogen oxides (NOx), particulate matter (PM), fuel economy and subsequently greenhouse gas (GHG) emissions while retaining high engine efficiency is a challenging task for the engine manufacturers. Around the world, heavy-duty vehicles is largely fueled with diesel fuel and accounts for very high transportcarbon emissions. For example, in the current European automobile market, heavy-duty diesel vehicles constitute for 25% of the CO2 emissions [1] and with predicted 45% increased demand by 2040, this percentage would likely escalate [2]. In recent years, it is well understood from the existing literature that there are not enough engine efficiency improvements of heavy-duty diesel vehicles. This is potentially due to the lack of major technological breakthrough [3,4]. Advanced combustion concepts such as low-temperature combustion (LTC) diesel [5,6], homogeneous charge compression ignition (HCCI) [7,8], reactivity controlled compression ignition (RCCI) [9,10] and partially premixed charge combustion (PPC) [11–17] has been investigated in detail to improve the thermodynamic efficiency due to reduced heat transfer losses and increased effective expansion ratio. However, the thermodynamic efficiency is not fully converted to brake thermal efficiency (BTE) because of the losses incurred during different cycle components of combustion, gas exchange, and mechanical efficiencies. To optimize the individual efficiencies, a wide variety of splitcycle engine concept has been investigated ranging from the “series XIV-engine” [18], the compact compression ignition (CCI) engine [19], the Scuderi engine [20], the “CryoPower” engine [21] and the double compression expansion engine (DCEE) [22]. The series XIV-engine comprises of two conventional combustion cylinders and one large low-pressure cylinder. Due to substantial heat losses, the engine efficiency was lower compared to conventional diesel engines. The CCI engine has an intake cylinder, a combustion cylinder and an exhaust cylinder. The simulations suggested that this concept can result in higher engine efficiency and lower heat transfer losses but the