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Technical Paper

Supercharged Homogeneous Charge Compression Ignition

The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here, a homogeneous charge is used as in a spark ignited engine, but the charge is compressed to auto-ignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOX formation. Earlier research on HCCI showed high efficiency and very low amounts of NOX, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas.
Technical Paper

Simultaneous PLIF Measurements for Visualization of Formaldehyde- and Fuel- Distributions in a DI HCCI Engine

Simultaneous laser induced fluorescence (LIF) imaging of formaldehyde and a fuel-tracer have been performed in a direct-injection HCCI engine. A mix of N-heptane and iso-octane was used as fuel and Toluene as fluorescent tracer. The experimental setup involves two pulsed Nd:YAG lasers and two ICCD cameras. Frequency quadrupled laser radiation at 266 nm from one of the Nd:YAG lasers was used for excitation of the fuel tracer. The resulting fluorescence was detected with one of the ICCD cameras in the spectral region 270-320 nm. The second laser system provided frequency tripled radiation at 355 nm for excitation of Formaldehyde. Detection in the range 395-500 nm was achieved with the second ICCD. The aim of the presented work is to investigate the applicability of utilizing formaldehyde as a naturally occurring fuel marker. Formaldehyde is formed in the low temperature reactions (LTR) prior to the main combustion and should thus be present were fuel is located until it is consumed.
Technical Paper

Simultaneous OH- and Formaldehyde-LIF Measurements in an HCCI Engine

Simultaneous OH- and formaldehyde LIF measurements have been performed in an HCCI engine using two laser sources working on 283 and 355 nm, respectively. Two ICCD camera systems, equipped with long-pass filters, were used to collect the LIF signals. The simultaneous images of OH and formaldehyde were compared with heat-release calculated from the pressure-trace matching the cycle for the LIF measurements. The measurements were performed on a 0.5 l single-cylinder optical engine equipped with port-fuel injection system. A blend of iso-octane and n-heptane was used as fuel and the compression ratio was set to 12:1. The width of the laser sheet was 40 mm and hence covered approximately half of the cylinder bore. At some 20 CAD BTDC low temperature reactions is present and formaldehyde is formed. The formaldehyde signal is then rather constant until the main heat-release starts just before TDC, where the signal decreases rapidly to low values.
Technical Paper

Optical Diagnostics of HCCI and UNIBUS Using 2-D PLIF of OH and Formaldehyde

Simultaneous OH- and formaldehyde planar-LIF measurements have been performed in an optical engine using two laser sources working on 283 and 355 nm, respectively. The measurements were performed in a light duty Diesel engine, using n-heptane as fuel, converted to single-cylinder operation and modified for optical access. It was also equipped with a direct injection common rail system as well as an EGR system. The engine was operated in both HCCI mode, using a single fuel injection, and UNIBUS (Uniform Bulky Combustion System) mode, using two injections of fuel with one of the injections at 50 CAD before TDC and the other one just before TDC. The OH and formaldehyde LIF images were compared with the heat-release calculated from the pressure-traces. Analyses of the emissions, for example NOx and HC, were also performed for the different operating conditions.
Technical Paper

Optical Diagnostics of HCCI and Low-Temperature Diesel Using Simultaneous 2-D PLIF of OH and Formaldehyde

Simultaneous OH- and formaldehyde planar-LIF measurements have been performed in an optical engine using two laser sources working on 283 and 355 nm, respectively. The engine used for the measurements was a car Diesel engine converted to single-cylinder operation and modified for optical access. The fuel, n-heptane, was injected by a direct injection common rail system and the engine was also fitted with an EGR system. The engine was operated in both HCCI mode and Diesel mode. Due to the low load, the Diesel mode resulted in low-temperature Diesel combustion and because of limitations in maximum pressure and maximum rate of pressure increase of the optical engine, the Diesel mode was run at a higher EGR percentage than the HCCI mode to slow down the combustion. A third mode, pilot combustion, was also investigated. This pilot combustion is created by an injection at 30 CAD before TDC followed by a second injection just before TDC.
Technical Paper

NOx and N2O formation in HCCI engines

Calculations using homogeneous and stochastic reactor models were performed in order to find an explanation to observed properties of NOx HCCI engines. It was found that for moderate NOx levels, N2O reactions play an important role in the NOx formation. Further, the high proportions of NO2 found in from some HCCI engines is due to high temperature inhomogeneities, poor mixing and slow overall combustion. N2O is often emitted from HCCI combustion. The levels of NOx in the exhausts are highly sensitive to temperature; however N2O has a weak negative dependence on temperature. While fuel rich operation naturally leads to high temperatures and thus high NOx levels; once the temperature effects are decoupled the fuel rich conditions themselves has a favorable effect on low-NOx engine operation.
Technical Paper

Evaluating the EGR-AFR Operating Range of a HCCI Engine

We present a computational tool to develop an exhaust gas recirculation (EGR) - air-fuel ratio (AFR) operating range for homogeneous charge compression ignition (HCCI) engines. A single cylinder Ricardo E-6 engine running in HCCI mode, with external EGR is simulated using an improved probability density function (PDF) based engine cycle model. For a base case, the in-cylinder temperature and unburned hydrocarbon emissions predicted by the model show a satisfactory agreement with measurements [Oakley et al., SAE Paper 2001-01-3606]. Furthermore, the model is applied to develop the operating range for various combustion parameters, emissions and engine parameters with respect to the air-fuel ratio and the amount of EGR used. The model predictions agree reasonably well with the experimental results for various parameters over the entire EGR-AFR operating range thus proving the robustness of the PDF based model.
Technical Paper

Cycle Resolved Wall Temperature Measurements Using Laser-Induced Phosphorescence in an HCCI Engine

Cycle resolved wall temperature measurements have been performed in a one cylinder port injected optical Scania D12 truck engine run in HCCI mode. Point measurements at various locations were made using Laser-Induced Phosphorescence (LIP). Single point measurements with thermographic phosphors utilize the temperature dependancy of the phosphorescence decay time. The phosphorescence peak at 538 nm from the thermographic phosphor La2O2S:Eu was used to determine temperature. A frequency tripled 10 Hz pulsed Nd:YAG laser delivering ultra violet (UV) radiation at 355 nm was used for excitation of the phosphor. Detection in the spectral region 535 - 545 nm was performed every cycle with a photo multiplier tube connected to a 3 GHz oscilloscope. Measurements were made at four points on the cylinder head surface and two points on the outlet and inlet valves respectively. For each location measurements were made at different loads and at different crank angle degrees (CAD).