The rapid depletion of petroleum fuel associated with its higher price and environmental legislation mandates use of environment friendly alternative fuels that replace diesel either partially or completely.
In HCCI engine operation, homogeneous air-fuel mixture prepared for auto-ignition by compression at number of locations within combustion chamber on reaching the chemical activation energy with chemical kinetics to control combustion process.
This technology provides higher BTE, ability to use variety of alternative fuels that includes diesel, hydrogen, natural gas, propane, butane, ethanol and dimethyl ether,, besides lower NOx and PM emissions.
Knocking due to premature combustion before the piston reaches the TDC, high levels of UHC and CO are inevitable with such engines.
Tests on engine with DF (Natural gas and diesel) either in CI or HCCI mode provided slightly higher BTE with advanced IT and also provided lower level of emissions.
NG and diesel fuel combination in HCCI engine provided BTE comparable to CI engine along with drastic drop in NOx and smoke emissions against high levels of HC and CO emissions.
It was also reported that for intake charge temperature between 80 and 135 °C BMEP ranged between 2.5 and 4 bar without misfiring and knocking.
Owing to high octane rating of NG, that made engine to operate at higher CR with lower emission level.
The NOx level lowered in NG HCCI operation showing further drop with deployment of EGR.
Admitted air temperatures with lower than 400 K is essential to initiate combustion for NG fuel at CR of 18.
The addition of H2 to NG improved HCCI engine performance and required less intake air heating.
The small amount of DME added to LPG controlled as per the study that revealed increased load range with a higher BTE and low level of NOx emission but increased UHC emissions.
HCCI engine resulted in higher PP, engine power and BTE for operation on higher quantity of H2 added to diesel with non-operation of engine with H2 as a sole fuel.
It was also reported that flammable mixture of H2 was in the range 0.14 (lean) to 10 (rich) with engine NOx emission increase on account of higher flame speed for H2.
The H2 content beyond 16% of the energy ratio lead to engine knock.
The HCCI operation utilizing H2 as a single fuel improved performance, with lean mixture too BTE improved significantly than CI mode of operation.
The rise in H2 content in NG fuel led to increased NOx emission.
With HCCI engine adopting H2 to control ignition evident through higher BTE with lower NOx emission.
Auto ignition reduced considerably due to utilization of OH by H2 when low temperature oxidation reactions of DME take place to reduce mixture OH radical concentration that retarded heat release delaying high-temperature oxidation process.
The retarded combustion phasing and reduced CD were resulted with H2 enrichment that yielded higher BP, BTE and stable combustion with narrow operational CR range and higher knock tendency.
The H2-diesel powered HCCI engine gave higher BTE as compared to the normal CI engine and the NG–diesel DF engine.
The future engines face the problem of Knock a major problem which if overcome make HCCI engines applicable to all kinds of vehicles and power generation.
Reduction in NOx, HC and CO emissions, by means of increasing the NG fraction, advancing the SOI, increasing the fuel fraction in ﬁrst DI injection with lower injection pressure and employing a wider injector spray angle was possible.
Better HCCI engine performance in terms of lower emissions with marginal loss in BTE with main IT of 20° before TDC was reported.
The strategies of different external and in-cylinder mixture preparation methods were discussed besides different strategies of controlled auto-ignition by HCCI combustion.
The paper reported the issues altogether by introducing HCCI engine control structure in progress.
It also revealed that research should result in appropriately controlled HCCI engines with acceptable performance and emissions characteristics at different loads.
The extensive literature review revealed that experimentation of use of BHO/BCO with H2 in HCCI mode to evaluate its performance has not been carried out for different HFER and load.
Hence the objective of the present experimental work is to study the performance, combustion and emission characteristics of HCCI engine powered with BHO/BCO and H2 fuel combinations for different HFER and load.
From the present experimental investigations on HCCI engine powered with BDFs and H2 combination following conclusions were drawn:
• HCCI engine powered with BDFs showed 2–3.4% lower BTE, 65–67% lower smoke, and 98–99% lower NOx emissions with HFER of 7% and EGR of 54% at 80% load as compared to CI mode.
• HCCI engine fuelled with BDFs resulted in about 11 times higher HC emissions with HFER of 7% and EGR of 54% at 80% load as compared to CI mode.
• HCCI engine powered with BDFs at same operating conditions showed CO emissions similar to CI mode at 80% load.
• Retarded combustion phasing with lower peak HRR was observed in HCCI combustion. HRR in HCCI operation with BDFs at 80% load was 50–52% lower as compared to CI mode of engine operation. PP was also lower by 28–31% in HCCI operation at the same conditions.
• Engine operation was not possible beyond 80% of full load due to severe knocking and maximum HFER substitution possible was below 8%.
On the whole, it can be concluded that CI engine operation with BDFs and hydrogen provides complete freedom from use of fossil fuels. These fuels are renewable and biodegradable. CI engine powered with these fuels require appropriate hardware modifications.
BDF powered engine operation with optimum engine operating parameters like IT, IP, nozzle geometry and CC shape showed overall better engine performance with reduced emissions. BDF powered CRDI engine yielded higher BTE and lower emissions compared to CI engine. BDF and Hydrogen was found to be a better option to operate the engine in HCCI mode to get improved BTE and lower exhaust smoke and NOx emissions.
References: S. V. Khandal, N.R. Banapurmath, V.N. Gaitonde, Performance studies on homogeneous charge compression ignition (HCCI) engine powered with alternative fuels, Renew. Energy. 132 (2019) 683–693. doi:10.1016/j.renene.2018.08.035.