Recent Trends In IC Engines

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A

PAPER PRESENTATION ON

“Homogeneous Charged Compression Ignition Engine”

Under the Heading

Recent Trend in I.C. Engine

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SUBMITTED BY

PRASAD V.

.


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INDEX:-

1. ABSTRACT

2. INTRODUCTION

3. HOMOGENEOUS CHARGE COMPRESSION IGNITION (THE HCCI PRINCIPLE)

o METHODOLOGY :

o REQUIREMENTS FOR HCCI :

o IMPORTANT TEMPRACTURES DIAGRAM:

o DIFFERENCE IN INITIATION OF COMBUSTION IN CI, SI & HCCI MODELS:

4. FORMATION OF COMBUSTIBLE MIXTURE

5.AUTO-IGNITION MECHANISM:

6.PARAMETERS AFFECTING HCCI COMBUSTION:

o In homogeneities:

o Equivalence (A/F) Ratio:


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o Temperature & pressure: Temperature: Pressure:

o Exhaust Gas Recirculation (EGR):

o Fuel composition/ octane number:

o Compression Ratio:

o Emissions:

7. ADVANTAGES OF HCCI:

8. BENEFITS:

9. CHALLENGS: CONTROLLING IGINITION TIMING OVER A RANGE OF SPEEDS AND LOADS:

COLD-START CAPABLITIES:

10. CURRENT APPLICATIONS IN HCCI TECHNOLOGY:

11.CONCLUSION:

12.REFERENCES:


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1. ABSTRACT

Diesel engines have wide range of utilization for automobiles, locomotives & marines

and co-generation systems. However, large problem is still related to undesirable emission.

I.C. engines must fulfill environment requirements, & deliver high power & torque, at low

fuel consumption & cost. Diesel engines produce high levels of NOx & particulate matter, spark

ignition engines have lower efficiency. Homogeneous charge compression ignition appears as a new

combustion technique that could combines in itself the advantages of engines, significant soot &

NOx reduction & low consumption at part load operations. This paper aims to give an overview of

this combustion mode.

HCCI has characteristics of the two most popular forms of combustion used in IC engines:

homogeneous charge spark ignition (gasoline engines) and stratified charge compression ignition

(diesel engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together.

However, rather than using an electric discharge to ignite a portion of the mixture, the density and

temperature of the mixture are raised by compression until the entire mixture reacts spontaneously.

Stratified charge compression ignition also relies on temperature and density increase resulting from

compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection

event, to initiate combustion.

HCCI engines have a long history, even though HCCI has not been as widely implemented as

spark ignition or diesel injection. It is essentially An Otto Combustion cycle. In fact, HCCI was popular

before electronic spark ignition was used. One example is the hot-bulb engine which used a hot

vaporization chamber to help mix fuel with air. The extra heat combined with compression induced

the conditions for combustion to occur. Another example is the "diesel" model aircraft engine.


http://en.wikipedia.org/wiki/Carbureted_compression_ignition_model_engines

http://en.wikipedia.org/wiki/Hot_bulb_engine

http://en.wikipedia.org/wiki/Ignition_system

http://en.wikipedia.org/wiki/Diesel_engine

http://en.wikipedia.org/wiki/Gasoline

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2. INTRODUCTION

Current pollution norms impose strict limitations on exhaust emissions. However

decreasing emissions while maintaining high efficiency is not simple task. In diesel engines,

compression ratios are high & combustion occurs by auto ignition. The temperature is high

giving rise to NOx emission, while soot formation takes place in fuel rich area in the

combustion chamber. In SI engines ignition released by means of a spark plug that ignites

the premixed homogeneous air-fuel mixture, & the A/F ratio is close to stoichiometry. In SI

engines there is problem of knocking, so in order to avoid severe temp. & pressure levels,

compression ratios are decreased, which deteriorates engine efficiency. Here NOx emissions

are relatively low, but an incomplete (partial) combustion results in un-burnt HC emissions.

HCCI combustion differs from SI combustion i.e. no flame front exists. Combustion takes

place at multiple sites in chamber due to high temp.

The defining characteristic of HCCI is that the ignition occurs at several places at a

time which makes the fuel/air mixture burn nearly simultaneously. There is no direct

initiator of combustion. This makes the process inherently challenging to control. However,

with advances in microprocessors and a physical understanding of the ignition process, HCCI

can be controlled to achieve gasoline engine-like emissions along with diesel engine-like

efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of

Nitrogen oxide emissions (NOx) without an after-treatment catalytic converter. The

unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak

temperatures), as in gasoline engines, and must still be treated to meet automotive

emission regulations.


http://en.wikipedia.org/wiki/Automobile_emissions_control


http://en.wikipedia.org/wiki/Catalytic_converter


http://en.wikipedia.org/wiki/Gasoline_engine

http://en.wikipedia.org/wiki/Control_theory

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3. HOMOGENEOUS CHARGE COMPRESSION IGNITION

THE HCCI PRINCIPLE

HCCI means that the fuel and air should be mixed before combustion starts and that the

mixture is auto ignited due to the increase in temperature from the compression stroke.

Thus HCCI is similar to SI in the sense that both engines use a premixed charge and HCCI is

similar to CI as both rely on auto ignition for combustion initiation. However, the

combustion process is totally different for the three types.

1. Methodology

2. Requirements of HCCI

3. Important Temperatures

METHODOLOGY


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REQUIREMENTS FOR HCCI

The HCCI combustion process puts two major requirements on the conditions in the cylinder:

(a) The temperature after compression stroke should equal the Auto ignition temperature of the fuel/air mixture.

(b) The mixture should be diluted enough to give reasonable Burn rate.

IMPORTANT TEMPERATURES

The figure below shows auto ignition temperatures for some fuels:

AUTO IGNITION TEMPERATURE FOR VARIOUS FUELS

It can be concluded that the minimum temp of 1000K should attained at end of compression thus fuel will get ignited. This temperature can be reached in two ways, either the temperature in the cylinder at the start of compression is controlled or the increase in temperature due to compression i.e. compression ratio is controlled. Rich mixture makes reaction to occur at very high rate which results in rise to extreme pressure rise rates and hence mechanical stress and noise. With a high auto ignition temperature like that of natural gas, it is also possible that formation of NOx can be the load limiting factor. Therefore the temperature should not rise beyond 1800K where above that NOx becomes significant. Partial oxidation of CO takes place at lower temp thus the ignition must raise the temperature above 1500K where CO is oxidized to CO2 in order to limit CO emissions.

Difference in initiation of combustion in CI, SI & HCCI modes:


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4. FORMATION OF COMBUSTIBLE MIXTURE :

A special issue of HCCI diesel engines, which is particularly important to achieve

stable combustion, is to find proper method for the fuel introduction. The process of

mixture formation can be performed in the cylinder internally or externally. The early fuel

injection accompanied by the fuel impingement on the cylinder walls, leading to very high

hydrocarbon where as the late fuel injection has to improved in order to obtain near

homogeneous mixture. As in the HCCI obtained from traditional SI engines, the diesel fuel

can be also premixed by applying an additional fuel injector in the intake port to obtain

external homogeneous mixture formation. The system allows high load operation in DI

mode without compromising performance, low to mid- load operation in HCCI mode, & in

region between where both system operates together.

5. AUTO-IGNITION MECHANISM:


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Mechanism of chain reaction:

Combustion phenomenon associated with chain reaction. Here free radicals react

with a molecule to produce a second free radical & a stable product & they govern rate at

which the chain continues. This step is called chain propagation step, reaction continue

consuming & producing radicals thus releasing heat, hence producing more free radicals

than consumed. These kinds of reaction results in explosion. When reactants are exhausted,

free radicals combine with one another to give stable molecules terminating the chain

reaction; & this step is called chain termination step.

Cool flame & negative temperature dependence of reaction rate:

After end of compression, an induction period occurs (t1) during which the pressure

remains sensibly constant. A cool flame then propagates through the mixture (first stage

heat release) & results in an increase in temp. & a corresponding increase in pressure. After

a further delay (t2), hot ignition occurs (second-stage heat release) result in a rapid increase

in pressure. The values of t1 & t2 depend on the nature of fuel & initial thermodynamic

conditions of the mixture as well as the equivalence ratio & t1 shows higher dependence on

temperature than t2.

The development of waves of cool flames is linked to the fact that the chemical reaction

accelerates in some volume elements more rapidly than elsewhere in the volume of

reaction mixture. Thus, ignition centre are formed, within which temp. & concentrations of

active species are high. Its appearance is characterized by sudden rise & fall of pressure.

Unlike main ignition, the cool flame process is hardly influenced by in homogeneities.

6. PARAMETERS AFFECTING HCCI COMBUSTION :


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In homogeneities:

Basically two types of in homogeneities considered inside the cylinder temp. & mixture (air-fuel) in homogeneity & that would have certain effects on combustion. For a perfectly homogeneous mixture the thermal efficiency is quite high with significantly low NOx, also the combustion duration would be short. But in case of temperature in homogeneity combustion duration become longer at the expense of lower thermal efficiency & higher NOx emission. The temperature In-homogeneity mainly originates from the mixing of the fresh charge with exhaust gas & wall heat transfer. If temperature in homogeneity become significant, then combustion is likely to be incomplete since colder parts of the mixture probably would not burn. Thus it is important to control the temperature In-homogeneity & intake temperature to achieve proper combustion without degrading thermal efficiency & without higher NOx. As far as combustible mixture is concerned higher the fuel in homogeneity lengthens combustion. In homogeneity in cylinder may have an ability of reducing pressure increase rate, max pressure & consequently knocking intensity, which means certain control can be established by introducing appropriate in- homogeneities to the in-cylinder mixture.

Equivalence (A/F) Ratio:

If the cylinder is hypothetically meshed into zones a as function of temp, high temp zones ignite before low temperature zones. Among zones within the same temp range, secondary effect then appears to be the equivalence ratio (a zone with higher F/A ignites before even if its T is slightly lower). Furthermore, equivalence ratio influences the combustion rate as well. Higher this ratio, higher will be the rate at which temperature increases & higher will be the peak value of temperature. Thereby combustion proceeds more rapidly.

Temperature & pressure:


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A] Temperature:

A reduction in initial temp delays the ignition tie significantly, slowing down the overall oxidation process. Higher the intake temp. Sooner will be the combustion. For low octane no. fuels, first-stage heat release is high, ignition is attained before TDC & increasing the intake charge temperature advances the ignition gradually; whereas for high octane number fuels first stage heat release is low, hence the temperature rises mainly because of compression work. Thus effect of intake temperature on phasing becomes more significant for these kinds of fuels.

B] Pressure:

For fixed intake charge temperature, boosting the charge the effect of advancing the ignition; in other words boosting reduces the need for higher temperature at the BDC.

Exhaust Gas Recirculation (EGR):

EGR can be considered as an inert gas that absorbs heat during his combustion reducing the combustion rate. It increases the inlet temperature when mixed with the fresh charge, decreases the O2 concentration, increases the heat capacity of the mixture owing to the high heat capacities of CO2 & water vapors (which reduces the temp. at the end of the compression) & introduces chemical species into fresh charge that will take part in chemical reactions.

A theoretical study realized using methane’s fuel (ON=120) & assuming the EGR composed of only CO2 ,H2O ,N2 &O2 showed that increasing the amount of internal EGR resulted in earlier combustion initiation for the temp. & pressure reached their peak values more rapidly.

Fuel composition/ octane number


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Start of ignition is very sensitive to the fuel octane number; Ignition delay increases with increasing octane number generally it is observed that ignition delay becomes shorter as the first stage is greater & this is highly dependent on the concentration of the fuel with low octane number. Latent heat of vaporization appears to influence auto ignition as well. Molecules with heat of vaporization absorbs a great quantity of energy during vaporization & therefore decrease the temperature of the mixture, this makes ignition more difficult & narrows the operating range at low loads, the effect inverses for high loads.

Compression ratio

It is observed that decreasing the compression ratio delays the ignition process. It also depends on the type of the fuel i.e. E.g. if engine uses diesel like fuels (n-heptanes) then compression ratio should be low, whereas for iso-octane requires higher compression ratio. Another thing is that at higher compression ratio HC emission increases.

Emissions

Because HCCI operates on lean mixtures, the peak temperatures are lower in comparison to spark ignition (SI) and Diesel engines. The low peak temperatures prevent the formation of NOx. This leads to NOx emissions at levels far less than those found in traditional engines. However, the low peak temperatures also lead to incomplete burning of fuel, especially near the walls of the combustion chamber. This leads to high carbon monoxide and hydrocarbon emissions. An oxidizing catalyst would be effective at removing the regulated species because the exhaust is still oxygen rich.


http://en.wikipedia.org/wiki/Diesel_engines

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7. ADVANTAGES OF HCCI :

The advantages of HCCI are numerous and depend on the combustion system to which it is compared.

1. Relative to SI gasoline engines, HCCI engines are more efficient, approaching the efficiency of a CIDI engine. This improved efficiency results from three sources: the elimination of throttling losses, the use of high compression ratios (similar to a CIDI engine), and a shorter combustion duration (since it is not necessary for a flame to propagate across the cylinder).

2. Relative to CIDI engines, HCCI engines have substantially lower emissions of PM and NOx. (Emissions of PM and NOx are the major impediments to CIDI engines meeting future emissions standards and are the focus of extensive current research.) The low emissions of PM and NOx in HCCI engines are a result of the dilute homogeneous air and fuel mixture in addition to low combustion temperatures.


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8. BENIFITS :

1. A major Benefit of HCCI combustion is its fuel-flexibility. In addition to gasoline, HCCI

operation has been shown for a wide-range of other fuels. Due to this fuel-flexibility, some

HCCI applications (e.g., light-duty vehicles) could use gasoline, while other HCCI applications

(e.g., heavy-duty trucks) could use diesel fuel.

2. CIDI emission control technologies are unproven, expensive, require the injection of fuel

or other reductions into the exhaust stream for NOx reduction, and currently do not last the

life of the engine. These emission control systems would also require the use of more

expensive ultra-low-sulfur fuels (less than 15 ppm).

9. CHALLENGS :

HCCI combustion is achieved by controlling the temperature, pressure and composition of

the air/fuel mixture so that it auto ignites near top dead center (TDC) as it is compressed by

the piston. This mode of ignition is fundamentally more challenging than using a direct

control mechanism such as a spark plug or fuel injector to dictate ignition timing as in SI and

CIDI engines, respectively. While HCCI has been known for some twenty years, it is only with

the recent advent of electronic engine controls that HCCI combustion can be considered for

application to commercial engines. Even so, several technical barriers must be overcome

before HCCI engines will be viable for high-volume production and application to a wide

range of vehicles


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1. Controlling Ignition Timing over a Range of Speeds and Loads:

Expanding the controlled operation of an HCCI engine over a wide range of speeds and loads

is probably the most difficult hurdle facing HCCI engines. HCCI ignition is determined by the

charge mixture composition and its temperature history (and to a lesser extent, its pressure

history). Changing the power output of an HCCI engine requires a change in the fueling rate

and, hence, the charge mixture. As a result, the temperature history must be adjusted to

maintain proper combustion timing. Similarly, changing the engine speed changes the

amount of time for the auto ignition chemistry to occur relative to the piston motion. Again,

the temperature history of the mixture must be adjusted to compensate. These control

issues become particularly challenging during rapid transients.

2. Cold-Start Capability:

At cold start, the compressed-gas temperature in an HCCI engine will be reduced because

the charge receives no preheating from the intake manifold and the compressed charge is

rapidly cooled by heat transferred to the cold combustion chamber walls. Without some

compensating mechanism, the low compressed-charge temperatures could prevent an HCCI

engine from firing.

10. CURRENT APPLICATION OF HCCI TECHNOLOGY:

Honda has recently demonstrated a concept for HCCI combustion for a production two

stroke engine and proved the reliability of this concept by competing in the Granada –Dakar

desert race with a pre-production motorcycle. A scooter, Honda Pantheon 125 cc, two

strokes, single cylinder equipped with HCCI engine is now commercially available on the

market.

GM has developed a new type of cylinder combustion process which improves fuel efficiency by 15% and is compatible with all commercially available gasoline and


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E85 fuels. Known as Homogeneous Charge Compression Ignition or HCCI, ignition is produced by compressing the air/fuel mixture in the combustion chamber without the use of a spark plug. The result is a flameless low-temperature burn which produces less CO2 and consumes less fuel. As a result, the HCCI engine has the efficiency of a diesel engine but without the NOx after treatment system. Despite the ability of the HCCI process to function without spark, it still needs heat to get started and therefore uses a conventional spark ignition system for cold starts, heavy engine load conditions and during speeds above 55 mph. The two test vehicles, Saturn Aura and Opel Vectra, used modified 2.2L Ecotec four-cylinder engines producing 180 hp and 170 lb/ft of torque (230 Nm) and maintain normal drivability except for a noticeable change in driving behaviour when the combustion process transitions from HCCI to conventional spark ignition. Like all new models, GM assures that further refinements will be made in all weather environments and the HCCI to spark transition will be unnoticeable.

11. CONCLUSION :

The major highlights related with the use of HCCI in automotive engines are that, the

ignition occurs at several places at a time which makes the fuel/air mixture to burn nearly

simultaneously. The process is inherently challenging to control. However, with advances in

microprocessors and a physical understanding of the ignition process, HCCI can be

controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency.

In fact, HCCI engines have been shown to achieve extremely low levels of emissions treated

to meet automotive emission regulations. Another major advantage related with the use of

this technology is the Fuel-flexibility that it offers.

So, in summary by considering today’s scenario, The HCCI technology is turned out to be a

boon for the rapidly growing automotive industry all-over the globe.




http://en.wikipedia.org/wiki/Gasoline_engine

http://en.wikipedia.org/wiki/Control_theory

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12. REFERENCES:

1. www.sciencedirect.com2. www.sae.org 3. Homogeneous Charge Compression Ignition – the Future ofIC engines? - Prof. Bengt Johansson INT. J.VEHICLEDESIGN VOL.44 Nos.1/2, 20074. Theoretical & experimental investigation on diesel HCCICombustion with external mixture formation. - MarcelloCanova, Midlam-Mohler, Yann Guezennec & Giorio Rizzoni.INT. J.VEHICLE DESIGN VOL.44 Nos.1/2, 20075. HCCI as an alternative combustion mode for the future ICEZeynep Serinyel, Luis Le Moyne & Philippe Guibert -INT. J.6. Wikipedia-the free encyclopaedia.


Recent Trends In IC Engines

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