The number of vehicles specifically in the transportation industry increases the energy demand.
The power demand of land vehicles are mostly provided by internal combustion engines.
The researchers are studying on alternative methods which will ensure a cleaner combustion due to depletion of petroleum energy resources in the near future and environmental pollution problems.
While some of researchers investigates the use of alternative fuels, some researchers have been focused on new combustion modes.
The usage of alternative fuels such as ethanol, methanol, biodiesel etc. have been taken attention widely till now.
However, constrained thermal efficiency of spark ignition engines (SI) because of limited compression ratio due to knocking and difficulty to reduce particulate matter (PM) and nitrogen oxide (NOx) emissions of compression ignition engines (CI) simultaneously enforce the researches to create alternative solutions to increase engine efficiency and reduce exhaust emissions.
Low temperature combustion modes such as homogeneous charge compression ignition (HCCI) [4,5], partially premixed compression ignition (PPCI) [6,7], reactivity controlled combustion ignition (RCCI) promising combustion modes providing higher thermal efficiency and simultaneously lower PM and NOx emissions.
Variable compression ratio (VCR) is another promising technology to run the engine at optimum point in terms of efficiency and emissions.
HCCI concept was firstly described by Onishi and Noguchi in 1979.
In HCCI mode homogeneous air fuel mixture is driven to the cylinder during intake stroke or fuel can be injected into the cylinder at early stage of compression stroke.
Even in both condition the mixture will be homogeneous before start of combustion (SOC).
From this aspect the preparation of the air fuel mixture is similar to the SI engines.
Combustion starts when homogeneous mixture is auto-ignited as in CI engines.
PM emissions is mostly caused by regional rich air fuel mixture.
PM emissions of an HCCI engine is reduced almost to zero level due to homogeneous and leaner air fuel mixture.
Leaner air fuel mixture also reduces the maximum in-cylinder temperature.
As a result of this aspect, NOx emissions are also decreased simultaneously with PM emissions.
Lower in-cylinder temperature decreases the heat loss to the cylinder wall.
Therefore, thermal efficiency of the engine increases.
On the contrary, HCCI engines have miscellaneous disadvantages such as high carbon monoxide (CO) and unburned hydrocarbon (HC) emissions, inability of controlling the combustion phase, knocking at high engine loads and having a limited operation range due to misfire at low engine loads.
The spontaneous combustion in-cylinder may lead to knocking especially at high loads [,,].
Stability, thermal efficiency and cyclic variations of the HCCI operation is mostly depended to SOC.
The main challenge of the HCCI operation is to control of SOC and combustion phasing because there is no any physical mechanism to ignite the mixture.
Therefore, most of the studies are focused on the effects of the parameters such as intake air temperature, research octane number, exhaust gas recirculation, injection timing etc.
Thermal efficiency of an internal combustion engine is mostly depended on CR of the engine.
Although upper limit of CR was restricted in SI engines because of knocking phenomena, high CR encourages the SOC in HCCI engines.
Beside that, CR can be used to control of SOC and combustion phasing for HCCI engines having VCR systems.
There are very less study investigating the effects of the CR on HCCI combustion parametrically and they are not including information about effects of the CR on operational range of the HCCI engine by means of engine speed and load.
Aceves et al. conducted a numerical study about effects of CR on methane HCCI combustion.
In the study ignition timing, combustion duration, indicated thermal efficiency and gross IMEP were examined for three different CR of 14:1, 16:1 and 18:1 at 1200 rpm engine speed.
It was reported that optimal operation regions providing high thermal efficiency are different for each CRs.
This result proves the requirement of VCR engines for varying operational conditions.
Pedersen and Schramm conducted a parametric study to understand the effects of the compression ratio, engine speed and equivalence ratio on HCCI combustion.
Dimethyl ether (DME) was used as fuel.
Experiments were performed at engine speeds of 1000, 2000 and 3000 rpm and with a sweep of excess air ratios of 2.5, 3 and 4.
It was found that higher compression ratios are required to achieve combustion at leaner excess air ratios.
Laguitton et al. investigated the effect of CR on exhaust emissions from a premixed charged compression ignition (PCCI) engine.
The aim of the study was to achieve emission target of Euro 6 with a low temperature combustion engine and reduce after treatment necessity.
Experiments were carried on with a single cylinder engine by reducing CR from 18.4 to 16.0.
It was determined that reducing CR or retarding injection timing reduced NOx emissions significantly while CO and HC emissions slightly increased.
Hadia et al. examined both the effects of CR and steam injection on performance, combustion and emission characteristics of an HCCI engine.
The study was performed numerically by varying CR between 15 and 20.
Three different steam injection ratios were investigated.
The results showed that burn duration, in-cylinder temperature and pressure increased by increasing CR.
CR of 18 and steam injection of 10% lead to decrease of CO by 40%.
Another study investigating the influence of CR on maximum load of an HCCI engine fuelled with natural gas performed by Olsson et al.
Experiments were performed with CRs of 15, 17, 20 and 21.
Results showed that the rate of heat release affected slightly by CR.
It was recommended to select CR to minimize NOx emissions at high loads.
Lida et al. conducted a study to investigate the effects of intake temperature, engine speed and compression ratio on HCCI engine operation using n-butane.
It was reported that operation range enlarged by means of engine speed as the CR increased.
It was also reported that the indicated specific fuel consumption decreased while equivalence ratio was increased.
The minimum indicated specific fuel consumption was obtained as 175 g/kWh.
An experimental study conducted by Kim et al. to investigate the effect of reduction of CR and spray injection angle on HCCI combustion.
Experiments were carried on a small direct injection diesel engine by reducing the CR from 17.8 to 15 via modifying the piston shape.
As a second approach injection timing was reduced from 156° to 60° to avoid fuel deposit on cylinder walls.
Conventional diesel combustion provided an IMEP value of 0.58 MPa.
After reducing CR IMEP value decreased from 0.58 to 0.55 MPa.
Machrafi and Cavadiasa performed an experimental and numerical study to present the influence of the inlet temperature, equivalence ratio and compression ratio on HCCI auto-ignition process by using primary reference fuels.
Inlet temperature was varied from 25 to 70 °C and equivalence ratio from 0.18 to 0.41.
CR was swept between 6 and 13.5 in the experiments and PFR40 and n-heptane were used as test fuels.
It was found that for a similar equivalence ratio, increasing compression ratio caused an increase in temperature and pressure in the cylinder.
As a result of this, reaction rate increased and ignition delay decreased.
Hyvonen et al. conducted a study on HCCI engine using variable compression ratio strategy.
In the study operation range of the engine by means of engine speed and load was examined with a naturally aspirated HCCI engine.
In the experiments commercial gasoline with a research octane number of 92 and primarily reference fuel having octane number 60 were tested at compression ratios of 14, 16, 18 and 20.
As a result of experiments it was reported that an operation range between 1000 and 5000 rpm engine speeds and 0–3.5 bar BMEP was obtained with commercial gasoline.
A similar combustion phasing study on HCCI engine was performed by Haraldsson et al..
In this study combustion phasing was tried to control by variable compression ratio equipment of the multi-cylinder test engine.
Additionally trade off between compression ratio and inlet air preheating was investigated.
It was found that the higher compression ratio can be used instead of preheating.
Engine speed range was determined as 1000–5000 rpm in a range of engine load idle to 4.4 bar BMEP.
However, BSFC maps were not presented in these studies.
Most of the previous studies regarding the effects of the CR on HCCI engine are parametric researches investigating the influence of CR on ignition delay, SOC, IMEP etc.
However operation range of the HCCI engine is another important factor that is limiting the usage of the engine.
Brake specific fuel consumption and operation maps are also important to control the engine while switching between HCCI mode to conventional SI or DI mode.
In the present study the effect of CR on in-cylinder pressure, rate of heat release (ROHR), SOC, combustion duration, indicated thermal efficiency, IMEP, pressure rise rate and HC, CO and NOx emissions at different lambda values and in the use of two different fuels of which research octane numbers were 20 (RON20) and 40 (RON40), were investigated parametrically.
Additionally and as a difference of this paper to previous studies, brake specific fuel consumption and engine operation maps were obtained by running the engine 169 experimental data points.
In this study a parametrical study was conducted to investigate the influence of the compression ratio on HCCI combustion performance and emissions.
In addition to parametric study, engine operational BSFC maps were obtained by running the engine at 169 experimental data point.
The experiments were conducted at different air/fuel ratios, fixed intake air temperature (353 K) and by using RON20 and RON40 fuels.
It was found that the increase of CR, advanced the combustion due to increasing pressure and temperature which propagated auto-ignition.
The increase of octane number of fuel along with CR led to maximum pressure and delayed heat release.
The in-cylinder chemical reactions highly decelerated when the mixture was excessively leaner, and this led to decrease of auto-ignition capacity.
This caused limiting HCCI combustion with misfire area.
Under same experimental conditions, the increase of octane number led to expansion of combustion duration.
Along with the increase of octane number, the combustion duration extended and therefore, the combustion was taken under control due to slow heat release.
While HC and CO emissions which are main problems of HCCI engines, decreased along with the increase of CR, NOx emissions which are low under operating conditions at low CR, increased a little along with the increase of CR.
Engine operation range enlarged with the increase of CR for RON40 fuel.
However widest operation range and minimum BSFC was obtained with RON20 and CR of 10.
Reference: A. Calam, H. Solmaz, E. Yılmaz, Y. İçingür, Investigation of effect of compression ratio on combustion and exhaust emissions in A HCCI engine, Energy. (2019) 1208–1216. doi:10.1016/j.energy.2018.12.023.