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An Experimental Study of HCCI Engine

Contents

Importance Working principle Starting of HCCI engines Control methods of HCCI Dual mode transitions Characteristics Recent developments Conclusion Reference

HCCI

Importance

SI engines have very low NOx and PM emissions

CI engines have high efficiency Homogeneous Charge Compression Ignition

(HCCI) is a promising alternative combustion

technology with high efficiency and lower NOx and particulate matter emissions

Principle

HCCI is characterized by the fact that the fuel and air are mixed before combustion starts and the mixture auto-ignites as a result of the temperature increase in the compression stroke

Optical diagnostics research shows that HCCI combustion initiates simultaneously at

multiple sites within the combustion chamber and that there

is no discernable flame propagation.

Comparison of SI and HCCI combustion

Spark Ignition HCCI

HCCI Concept

HCCI POTENTIAL1. High efficiency, no knock limit on compression

ratio.2. Low NOx and no NOx after treatment systems

required.3. Low PM emissions, no need for PM filter.4. HCCI provides up to a 15-percent fuel savings,

while meeting current emissions standards. 5. HCCI engines can operate on gasoline, diesel fuel,

and most alternative fuels. 6. In regards to CI engines, the omission of throttle

losses improves HCCI efficiency.

HCCI

BARRIERS1. The auto-ignition event is difficult to

control, unlike the ignition event in spark -ignition(SI) and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively.

2. HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions

3. High HC and CO emissions.

Starting HCCI engines

Charge does not readily auto ignite cold engines.

Early proposal was to start in SI mode and run in HCCI mode.

It involves the risk of knocking and cylinder failure at high compression ratios.

Now intake air pre-heating with HE and burner system allows startup in HCCI mode with conventional starter.

Starting HCCI engines

Control methods of HCCI combustion

The spontaneous and simultaneous combustion of fuel-air mixture need to be controlled.

No direct control methods possible as in SI or CI engines.

Various control methods are:

Variable compression ratio

Variable induction temperature

Variable valve actuation

Control methods of HCCI combustion

Variable compression ratio method The geometric compression ratio can be changed

with a movable plunger at the top of the cylinder head. This concept used in “diesel” model aircraft engine.

Variable induction temperature The simplest method uses resistance heater

to vary inlet temperature. But this method is slow

Now FTM (Fast Thermal Management) is used. It is accomplished by rapidly varying the cycle to cycle intake charge temperature by rapid mixing.

FTM systemRapid mixing of cool and hot intake air takes place achieving optimal temperature as demanded and hence better control.

FTM Control methodCombustion timing can be controlled by adjusting balance of hot and cold flow

Variable valve actuation (VVA) This method gives finer control within

combustion chamber Involves controlling the effective pressure

ratio. It controls the point at which the intake valve closes. If the closure is after BDC, the effective volume and hence compression ratio changes.

Control methods of HCCI combustion

Control methods of HCCI combustion

Dual mode transitions

When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures

This restricts engine operation at high loads.

Dual mode transitions

Practical HCCI engines will need to switch to a conventional SI or diesel mode at very low and high load conditions due to dilution limits

Two modes: HCCI-DI dual mode HCCI-SI dual mode

SI mode transitions

It equips VVA and spark ignition system Operates in HCCI mode at low to medium

loads and switches into SI mode at higher loads

Transition is not very stable and smooth

DI-HCCI

Long ignition delay and rapid mixing are required to achieve diluted homogeneous mixture.

Combustion noise and NOx emissions were reduced substantially without an increase in PM.

Combustion phasing is controlled by injection timing.

Thus DI-HCCI proves to be promising alternative for conventional HCCI with good range of operation.

Combustion characteristics

Emission characteristics

Emission characteristics

Emission characteristics

Engine performance

Engine performance

Recent developments in HCCI

Turbo charging initially proposed to increase power Challenges for turbo charging1. Exhaust gas temperatures low (300 to 350 °c)

because of high compression ratio.2. Post turbine exhaust gas temperature must be high

enough to preheat intake fuel-air mixture in HE.3. Low available compressor pressure ratio.

Recent developments in HCCI

Solution for turbo charging1. Use VGT (Variable Geometry Turbine) which allows

for a greater range of turbine nozzle area, better chance to achieve high boost.

2. Combining turbo charging and super charging may be beneficial.

EGR (Exhaust Gas Re-circulation) Can be adopted for higher efficiencies and lower HC and CO emissions.

Recent developments in HCCI

The exhaust has dual effects on HCCI combustion.

It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine work.

Reduce the CO and HC emissions.

HCCI prototypes

General Motors has demonstrated Opel Vectra and Saturn Aura with modified HCCI engines.

Mercedes-Benz has developed a prototype engine called Dies Otto, with controlled auto ignition. It was displayed in its F 700 concept car at the 2007 Frankfurt Auto Show

Volkswagen are developing two types of engine for HCCI operation. The first, called Combined Combustion System or CCS, is based on the VW Group 2.0-litre diesel engine but uses homogenous intake charge rather than traditional diesel injection. It requires the use of synthetic fuel to achieve maximum benefit. The second is called Gasoline Compression Ignition or GCI; it uses HCCI when cruising and spark ignition when accelerating. Both engines have been demonstrated in Touran prototypes, and the company expects them to be ready for production in about 2015.

HCCI prototypes

In May 2008, General Motors gave Auto Express access to a Vauxhall Insignia prototype fitted with a 2.2-litre HCCI engine, which will be offered alongside their ecoFLEX range of small-capacity, turbocharged petrol and diesel engines when the car goes into production. Official figures are not yet available, but fuel economy is expected to be in the region of 43mpg with carbon dioxide emissions of about 150 grams per kilometre, improving on the 37mpg and 180g/km produced by the current 2.2-litre petrol engine. The new engine operates in HCCI mode at low speeds or when cruising, switching to conventional spark-ignition when the throttle is opened

Conclusions

HCCI-DI combustion with n-heptane/diesel dual fuel is a 3-stage combustion process consisting of cool flame, HCCI combustion and diffusive combustion.

Increase of premixed ratio, shortens the NTC, increases the peak in-cylinder pressure and temperature and rises the highest heat release rate of HCCI combustion phase

Conclusions

NOx emissions decrease firstly at low premixed ratios and exhibit a trend of increasing at higher premixed ratios.

Pre-mixed ratio has no significant effect on soot emission and the soot emission could remain at the same level but then have a peak value with a certain higher premixed ratio relating to the equivalence ratio.

Conclusions

The change of CO with premixed ratio is mainly depending on whether the premixed equivalence ratio exceeds the critical value. UHC increases almost linearly with the premixed ratio mainly due to the incomplete oxidation in the boundary layer and the crevices.

Conclusions

The IMEP increases with the increase of premixed ratio at low to medium loads.

The indicated thermal efficiency shows deterioration at high load with large premixed ratios.

Reference

http://www.wikipedia.com ^ Zhao, Fuquan; Thomas W. Asmus, Dennis N. Assanis, John E. Dec,

James A. Eng, Paul M. Najt (2003). Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and Development Issues. Warrendale, PA, USA: Society of Automotive Engineers. pp. 11–12. ISBN 076801123X

http://www.howstuffworks.com ^ Pitz, William J.; SM Aceves, JR Smith, CK Westbrook (1999).

"Compression ratio effect on methane HCCI combustion". JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME 212 (3): 569-574. 

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