This promising alternative to combustion engines needs more development before mass production.
By Dr. Raj Shah, Dr. Vikram Mittal, Shi Ying Zheng
Homogeneous charge compression ignition (HCCI) engines have better gas mileage and lower nitrogen oxide (NOx) and particulate matter (PM) emissions than traditional diesel or spark-ignition engines, but there are challenges that need to be addressed before potential broad-scale use in commercial vehicles.
A traditional gasoline spark-ignition engine is characterized as a flame propagation mechanism. Variable ignition time from the spark discharge can influence the start of combustion in the engine. In the spark-ignition engine, the electric control unit sets the ignition timing as per the ignition pattern.
A compression-ignition engine combustion is conventional diesel combustion, which occurs at higher compression ratios than spark-ignition engines. Rather than being ignited by a spark plug, the air-fuel mixture in this engine autoignites as a result of piston compression.
The HCCI process is similar to that of a spark-ignition engine, which uses a homogeneous charge for combustion, and to that of a compression-ignition engine, which uses autoignition. As a result, HCCI is a combination of spark-ignition and compression-ignition combustion processes. However, HCCI engines differ from conventional ICEs in that there are no spark plugs or injectors to assist combustion. The HCCI engine has the high efficiency of compression ignition and the very low emissions of spark ignition with catalytic converters. HCCI is also a single-fueled engine that applies a homogeneous charge to premixed fuel and combusts the mixture.
In an HCCI engine, gasoline is injected into warmed air by port fuel injection or direct injection in the cylinder to establish a homogenous charge. The temperature, pressure and concentration origins of the homogeneous charge, as well as the specific reaction mechanism of the fuel, air and residual gas mixture, all influence the ignition timing of an HCCI engine. First, fuel and air are premixed at the beginning of the combustion process. During the compression stroke, the homogenous charge is heated, hits the autoignition point at the TDC, and then autoignites in many points throughout the cylinder. The mixture also autoignites when the temperature rises in the compression stroke.
HCCI Engine Advantage
HCCI engines could be a new alternative to conventional combustion engines. Compared with diesel combustion, which relies on a fuel/air mixture, HCCI reactions are not always constrained by the mixing speed at the fuel jet-oxidizer contact. HCCI engines lack a visible flame front and a confined high-temperature reaction area, compared with conventional spark-ignition combustion, which primarily depends on flame propagation. HCCI combustion is a chemical kinetic reaction regulated by temperature, pressure and in-cylinder charge composition, which reacts faster than both spark ignition and compression ignition. Additionally, HCCI combustion reaction usually happens quickly at low temperatures, and the bulk energy released at high temperatures is controlled by carbon monoxide oxidation. This results in limited combustion reactions occurring at high temperatures, which reduces the amount of emitted CO2.
HCCI engines have been certified to emit low NOx and particulate matter and produce high thermal efficiency. HCCI combustion can reduce NOx emissions by 90-98% compared to traditional compression-ignition combustion. This is because of the absence of high-temperature regions within the combustion chamber in the mechanism of the HCCI engine, as noted in the SAE International technical paper “Homogeneous Charge Compression Ignition (HCCI): Benefits, Compromises, and Future Engine Applications”. The global fuel/air ratio in HCCI combustion processes is normally relatively low, and the temperature within the reaction chamber is substantially lower than in compression-ignition or spark-ignition engines. So, the time required for full homogeneous mixing of air and fuel under low-load circumstances is less than the ignition delay period. HCCI combustion also produces low levels of smoke due to the lack of diffusion-limited combustion and confined fuel-rich zones. However, poor mixture preparation may form liquid fuel deposition during the combustion cylinder and in localized fuel-rich areas as the SAE technical paper notes.
HCCI Challenges and Solutions
Even though the HCCI engine has better thermal efficiency and emissions than conventional engines, it has various restrictions and technical challenges. Homogenous charge preparation, controlled auto-ignition timing, combustion phase, high-pressure rise rate, knocking and noise, operation range limitation over a wide speed and load, high HC and CO, and cold start all must be resolved before mass production and release on the market.
The two main stumbling blocks to adoption are controlling the ignition timing and combustion phase. HCCI combustion has a low processing range and lacks control over the ignition process. Unlike traditional engines, HCCI engines do not have a direct strategy to control the start of combustion. The HCCI combustion process starts when the air-fuel mixture auto-ignites, which the mixture is exposed as fuel burns and time-temperature history reacts. The combustion phasing of HCCI engines is influenced by fuel autoignition qualities, fuel content, residual rate and reactivity, mixture homogeneity, compression ratio, intake temperature, fuel latent heat of vaporization and engine temperature, heat flux to the engine and other engine-dependent factors.
In addition, during the combustion phase in the cylinder, the combustion rate and combustion pressure heating rate are critical factors. High-pressure increase rates cause severe sonic vibrations in the cylinder, resulting in knocking combustion and risking engine damage. To control HCCI combustion phasing, the most important strategy is to control the time-temperature relation. This leads to changing the behavior of the mixture for autoignition.
Limited operating load range is one of the biggest barriers to successful development. Controlling charge autoignition at greater loads is challenging, and since there is no direct way of controlling combustion at greater engine loads, it is hard to manage the frequency of combustion. To solve these challenges, an engine can be run in several modes: spark-ignition and compression-ignition modes for one-third to one-half of the operating load range, and HCCI mode for the balance. Researchers have further discovered that by substantially stratifying the charge (temperature and mixture stratification) under high engine loads to prolong the heat-release process, the operational range may be greatly increased.
Use in Hybrid Vehicles
There is the potential to use HCCI in hybrid vehicles to limit the operating range of the engine so that autoignition can be controlled. Ignition control challenges stem from the lack of a direct trigger, as well as the narrow working range between the upper “knock” and lower “misfire” limits. To help this, dual-fueled combustion technology was developed as an alternative to better regulate the combustion phase by combining two different reactivity fuels while retaining low NOx and particulate matter emissions. Thus, an HCCI engine with a dual-mode operation (e.g., SI-HCCI) will be essential to cover the entire operating range in an automobile application. The dual-mode engine with a hybrid-electric powertrain can extend engine duration when operating in the HCCI mode and reduce transitions between HCCI and spark-ignition modes.
Hybrid electric vehicles (HEVs) improve fuel efficiency without losing vehicle performance or usability. For example, HCCI hybrid spark ignition can replace the standard spark ignition in a traditional vehicle, which improves the fuel economy by about 17%. In addition, HEVs are available in a wide range of configurations and levels of electrification. Usually, the most fuel-efficient choice for a dual-mode system would be a current compression-ignition engine, but combining a highly expensive engine with hybrid results in an unaffordable vehicle price.
Emissions, Noise and Exhaust
HCCI engines normally emit higher HC and CO than traditional diesel engines. As is true with all homogeneous combustion processes, a large percentage of the in-cylinder fuel is retained in fissures during the compression stage and avoids combustion. Also, the temperature is not high enough to burn most of the unreacted gasoline when it flows back to the cylinder during the adiabatic expansion. Therefore, both HC and CO emissions boost massively due to incomplete combustion.
The knock tendency is affected by the two fuels under varying load conditions, as represented by the fuel energy absorbed in the autoignited zone. Knocking occurs when the piston does not reach TDC due to early combustion. Furthermore, the degradation of combustion efficiency along with ignition issues influences the performance of HCCI combustion at the lowest loads. At greater loads, the rate of pressure sharply increases, which causes engine noise to rise dramatically, and if left unchecked, the engine might fail.
Since the exhaust gas includes more CO2 than O2, engine-out exhaust gas recirculation (EGR) raises the combustion temperature dramatically. As a result, the EGR principle demands combining the remaining gases from the prior period with fresh charge or air in the inlet manifold to reduce NOx emissions. EGR improves the charge’s autoignition capabilities by lowering the maximum pressure during heating.
Internal EGR, which may be adjusted by variable valve timing, improves cold start and warmup times. In a study using a single cylinder spontaneously released HCCI engine with both internal and external EGR, researchers found that when negative valve overlapping increased, NOx emissions rose according to the high temperature of the gases, whereas external EGR had no impact on NOx emissions. For best outcomes, EGR should be combined with another innovation such as alternative fuel or a chemical method.
Advanced combustion technologies have been studied extensively to replace conventional combustion engines. HCCI engines not only improve effective efficiency and reduce emissions of NOx and particulate matter compared with the internal combustion engine but also display a high level of adaptation and flexibility to many types of fuels. However, the HCCI engine is not perfect; it has challenges affecting its successful operation. Future studies should aim to overcome those difficulties before widescale manufacturing of HCCI engines for commercial use. The main problems are combustion phasing control, high HC and CO emissions, operation range extension, cold start and homogenous mixture preparation.
Dr. Raj Shah is a director at Koehler Instrument Company. He is an elected Fellow by his peers at IChemE, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook.”
Dr. Vikram Mittal, is an assistant professor at the United States Military Academy in the Department of Systems Engineering. Mittal’s current research interests include system design, model-based systems engineering and modern engine technologies.
Shi Ying Zheng is a chemical engineer student from SUNY, Stony Brook University, where Drs. Shah and Mittal are on the external advisory board of directors.
