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Proceedings of TFMS 2012
National Conference on Thermal, Fluid and Manufacturing Science
January 2021, 2012
Surat, Gujarat, India
Combustion Control with Trapped Residual Gas
(TRG) for 4-Stroke HCCI/CAI EnginesSantosh B Trimbake
Assistant Professor
Department of Mechanical Engineering
College of Military Engineering Pune Maharashtra 411031
santoshtrimbake @ yahoo.co.in
Mobile No: +919960431466
Abstract: Over the last decade, an alternative combustion
technology, commonly known as homogeneous charge compression
ignition (HCCI) or controlled auto-ignition (CAI) combustion, has
emerged that has the potential to achieve efficiencies in excess of
GDI engines and approaching those of current CI engines, with very
low NOx emissions and virtually no smoke emissions. While thepotential benefits of this new combustion technology are significant,
this combustion mode faces its own set of challenges, such as
difficulty in controlling the combustion phasing, a restricted
operating range, and high hydrocarbon emissions.
The most successful and practical approach to HCCI/CAI
combustion initiation and control in a gasoline engine is through the
use of large amounts of burned gases by trapping them within the
cylinder or through internal recirculation, as their thermal energy
will heat the charge to reach auto-ignition temperature and help totame the heat release rate.
This paper presents the review of combustion control
technology with residual gas trapping using variable valve actuation
for 4 stroke HCCI/CAI engine.
Advancements in combustion control with residual gas
trapping technology has been reviewed as reported in the literature,
as it can realized in both single/multi-cylinder production type PFI
(Port Fuel Injection) and DI (Direct Fuel Injection) gasoline engines
using the NVO (negative valve overlap) approach.
Initially the various aspects of residual gas trapping for
PFI gasoline engine have been presented. Subsequently, DI which
offers more independent control over CAI output and combustion
phasing, has been discussed. Further how fuel injection timing canbe used as an effective means to control CAI combustion has been
evaluated.
Keywords: CAI, HCCI, PFI, DI, Control, NVO & Residual
gas trapping
1 INTRODUCTION: HCCI is an alternative piston-engine
combustion process that can provide efficiencies as high as CI,
engines while, unlike CI engines, producing ultra-low NOx and PM
emissions. HCCI engines operate on the principle of having a dilute,
premixed charge that reacts and burns volumetrically throughout the
cylinder as it is compressed by the piston. In some regards, HCCI
incorporates the best features of both SI and CI. Most engines
employing HCCI to date have dual mode combustion systems inwhich traditional SI or CI combustion is used for operating
conditions where HCCI operation is more difficult. Typically, the
engine is cold-started as an SI or CI engine, then switched to HCCI
mode for low- to mid-load operation to obtain the benefits of HCCI
in this regime, which comprises a large portion of typical automotive
driving cycles. For high-load operation, the engine would again be
switched to SI or CIDI operation. Research efforts are underway to
extend the range of HCCI operation .Combustion control is the
biggest challenge to HCCI engines becoming a commercial success.
Trapped Residual gas method (TRG) with variable valve actuation
(VVA) seems to be one of the most effective and practical
combustion control approach for four-stroke gasoline engines(PFI/DI)
2 FUNDAMENTALS OF CAI/HCCI GASOLINE
ENGINES: CAI combustion is achieved by controlling the
temperature, pressure and composition of the air/fuel mixture so that
auto-ignited combustion can start at the right time and will proceed
without causing a runaway heat release rate. There is no direct
control over the ignition timing as in a SI or CI engine In an ideal
case, CAI/HCCI combustion can be described as controlled auto
ignition of a premixed fuel/air mixture and involves the simultaneous
reactive envelopment of the entire fuel/air mixture without a flame
front. As shown fig1, the initiation of combustion always occurs at
multiple sites in the premixed fuel/air mixture. The heat release
process is much faster than the conventional SI combustion and is
more closely described by a constant volume heat addition process,
This combustion mode also results in a more uniform and repeatable
heat release incomparison with that of SI operation. The cumulative
heat release in such an engine is therefore the sum of the heat
released Q from the complete mixture in the cylinder, m, each
combustion reaction, dq i and where k is the total number of heat
release reactions, and q is the heat released from the i th heat release
reaction involving per unit mass of fuel and air mixture (fig 2).
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2.1 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.
Fig 1 HCCI combustion process
Fig 2 heat released in HCCI combustion
3 CHALLENGES FOR HCCI COMBUSTION [1]: HCCI
combustion is achieved by controlling the temperature, pressure, and
composition of the fuel and air mixture so that it spontaneously
ignites in the engine. This control system is fundamentally morechallenging than using a spark plug or fuel injector to determine
ignition timing as used in SI and CI engines, respectively. The recent
advent of electronic engine controls has enabled consideration of
HCCI combustion for application to commercial engines. Even so,
several technical barriers must be overcome to make HCCI engines
applicable to a wide range of vehicles and viable for high volume
production. Significant challenges include:
Controlling Ignition Timing and Burn Rate Over a Rangeof Engine Speeds and Loads
Extending the Operating Range of HCCI to High EngineLoads
Cold-Starts and transient response with HCCI Engines
Minimizing Hydrocarbon and Carbon Monoxide Emissions4 ADVANCEMENTS IN COMBUSTION CONTROL
TECHNOLOGIES [1]: Combustion control is the biggest
challenge to HCCI engines becoming a commercial success. For this
reason, several methods have been proposed for achieving HCCI
engine control over the wide range of operating conditions required
for typical transportation-engine applications. Control technologies
reported in the literature have demonstrated some degree of success.
Some of the proposed methods include:
Trapped Residual Gas (TRG) using VVA(Variable valve actuation): Here residual gas from the
previous cycle is trapped for next cycle by VVA and used
as driving force for charge temperature to attained auto
ignition temperature Amount and temperature of TRG is
the tool to control the combustion.
Variable compression ratio (VCR): VCR engine hasthe potential to achieve satisfactory operation in HCCImode over a wide range of conditions because the
compression ratio can be adjusted as the operating
conditions change.
Thermal control In this methodology, thermal energyfrom exhaust gas recirculation (EGR) and compression
work from a supercharger are either recycled or rejected to
obtain satisfactory combustion.
Ignition-enhancing additives HCCI engine controlcould be achieved by using two fuels with different octane
ratings.
5. PRINCIPLE OF CAI/HCCI OPERATION WITH
RESIDUAL GAS TRAPPING: The principle of CAI operationwith residual gas trapping is to initiate CAI combustion and to
control the subsequent heat release rate by trapping large and variable
amounts of residual gases in the cylinder. The burned gases from the
previous cycle are trapped in the cylinder by closing the exhaust
valves relatively early. Unlike spark ignition operation, the engine
load is controlled primarily by the exhaust valve timing/lift. As the
load is decreased, the exhaust valve closure (EVC) is advanced so
that more burned gases are trapped, and the intake valve opening
(IVO) is retarded accordingly to avoid backflows of residuals into the
intake ports. This leads to negative valve overlap, NVO (fig 3)
which is in contrast to positive valve overlap normally used in SI
engines in order to maximize the volumetric efficiency Combustion
control by retained residual gas is often called controlled auto-
ignition, CAI. Fig 4 shows how residual gas can be retained using a
NVO and fig 5 shows the valve timing diagram for residual gas
trapping .
The larger amount of residuals trapped will lead to less
fresh charge being admitted into the cylinder and hence less fuel
being burned. Conversely, retarded EVC will lead to reduced amount
of residuals and hence more fresh charge to be flowed into the
cylinder for greater work output. It is important to note that such
operations are carried out with wide open throttle and hence there are
no pumping losses associated with a partly closed throttle as in the SI
operation.
From above discussion it can be realized that Variablevalve actuation (VVA) needs to be used to control the timing / lift
of exhaust valve and inlet valve to control the initial charge
temperature by retaining variable amount of residual gas.
VVA exists in many flavors with different degrees of
freedom. VVA could be implemented in an engine with mechanical,
magnetic, or hydraulic valve actuators. There are systems that merely
provide cam phasing on the intake/exhaust cam shaft. Other systems
provide cam profile switching, CPS, or combinations of cam phasing
and CPS. Finally there are systems that provide fully variable valve
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timing as well as lift (electro-hydraulic / electro-magnetic VVA
system).
The NVO approach is attractive since many production
engines have VVA capability that allows CPS and cam phasing.
Fig 3 Illustration of NVO strategy (dashed) as opposed to the
normal positive valve overlap strategy (solid).
Fig 4 In-cylinder P- diagram with residual gas trapping
Fig 5 Valve timing diagrams for different operation strategies.
6. CAI OPERATION IN A FOUR-STROKE PORT FUEL
INJECTION (PFI) GASOLINE ENGINE
J. Li & et. al. [2] first investigated the performance and emission
characteristic on 4-cylinder production type PFI gasoline engine
using substantially standard components, modified only in cam
dimensions to control the gas exchange process in order tosignificantly increase the trapped residuals. operating with CAI and
equipped with Variable Cam Timing (VCT) .The engine used was a
Ford 1.7 Litre Zetec SE 16-valve 4-cylinder PFI gasoline engine with
sequential fuel injection strategy to ensure that the same mixture
preparation event was applied to each of the four cylinders The intake
and exhaust camshafts were equipped with two independent VCT
system with a pair of special camshafts of reduced lift and the
compression ratio was kept at 10.3. The fuel used was the standard
unleaded gasoline of RON 95. During the tests the throttle was kept
at wide open and the air flow was changed by varying the cam
timings, which could be continuously changed by up to 40 degrees
crank angle. All experiments were carried out when the coolant
reached 90 0 C or over in order to minimize the effects of coolant
temperature.
6.1Performance and combustion characteristics
It was found that the largely increased trapped residuals alone were
sufficient to achieve CAI in this engine and with VCT, a range of
loads between 0.5 and 4 bar BMEP and engine speeds between 1000and 3500 rpm ( fig 6) were mapped for CAI fuel consumption and
exhaust emissions. The measured CAI results were compared with
those of Spark Ignition (SI) combustion in the same engine but with
standard camshafts at the same speeds and loads. There was a linear
correlation between the residual fraction and engine output,
independent of the engine speed (fig 7). The higher the residual
fraction was, the lower the torque became. As the engine was
operated at WOT, the mass in the cylinder was more or less the same
and only the mixture concentration changed .Fig 8 shows variation
of the maximum rate of pressure rise with residuals or load. In most
cases, the maximum rate of pressure rise decreased with residuals or
increased with load, so was the peak cylinder pressure. The rate of
maximum pressure rise varied between 1 and 7 bar/CA.
Fig 6 CAI operation range with residual gas trapping
6.2Fuel consumption and emission characteristics
Fig 9 to12 compare the fuel consumption and emission results of the
CAI combustion mode and SI mode from the same engine. The
comparison showed more than 30% reduction in BSFC (Fig 9) is
seen . Up to 99% reduction in NOx at low loads(fig 10) is seen with
CAI operation But it should be noted that CAI combustion in the 4-
stroke gasoline engine had been always associated with higher CO
emissions than the SI combustion until the residual gas trapping
method was employed.(fig 12). Fig 11 shows that the unburned HCs
were much higher from CAI combustion than that from SI
combustion with port-fuel injection, but they were on a par with those
from the stratified charge direct injection gasoline engine
7 EFFECT OF DI ON CAI COMBUSTION IN THE 4-
STROKE GASOLINE ENGINE During the CAI operation,
engine output is principally controlled by the EVC timing. As the
engine load or speed increases, combustion starts earlier and burns
faster, leading to too rapid a rate of heat release and very high peak
cylinder pressure as well as higher fuel consumption.
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Fig 7 Effect of residual fraction on maxi. Rate of Pressure rise.
Fig 8 Relationship between residual fraction & engine o/p
Fig 9 Changes in BSFC (%) with CAI combustion relative to SI.
Fig 10 Changes in BSNOx emissions (%) with CAI combustion
At the low load region, the very retarded combustion
causes large cyclic variation and even partial burn therefore, it will be
desirable and necessary to find other means capable of more
independent control over the combustion phasing from the engine
load, in order to improve the CAI combustion and its operational
range Research done at the Brunel Universitys laboratory [35] has
shown that direct fuel injection into the cylinder can be used as one
of the most effective means of controlling the combustion phasing for
optimised engine performance and emissions. This section will
present some of the main findings from such studies
Fig 11 Changes in BSHCs emissions (%) with CAI combustion
relative to SI.
Fig 12 Changes in BSCO emissions (%) with CAI combustion
relative to SI.
There are three significant phases occurring sequentially in the CAI
engine cycle with negative valve overlap, namely, residuals
trapping, residuals conditioning and CAI combustion. Each phase
has an effect on the following, with the end-of-cycle conditions
feeding back to the first phase in order to sustain the CAI operation
continuously.
The residual trapping phase is controlled primarily by the exhaustvalve closing timing (variable early EVC) and the trapped residuals
temperature Once trapped, the mass of the residuals is fixed for the
subsequent cycle, but its temperature and pressure are variable during
theresiduals conditioning phase according to re-compression and re-
expansion and further heat subtraction and heat addition if fuel can be
injected directly into the residuals during that period. At the end of
the residuals conditioning phase which also marks the beginning the
intake period (variable late IVO), the released temperature and
pressure of the residuals will affect the intake air flow, the charge
dilution (residuals/total volume ratio) and the combustible charge
temperature, while direct fuel injection during the intake and
compression periods will affect the charge homogeneity or
stratification as well as charge temperature and quantity, leading up
to the CAI combustion phase which is characterized by the CAI
ignition timing, heat release rate, IMEP, ISFC and exhaust emissions.
The final exhaust gas temperature after CAI is then fed back to the
next cycle to initiate the next residuals trapping phase
Initially single fuel injection timing strategy has been used to study
its effect on combustion characteristics which has been further
grouped into three categories [7] :
( i ) Early injections during the negative valve overlap period, in
which fuel is injected into the hot residual gas in the cylinder for the
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purpose of reforming the fuel or initiating the minor combustion if
possible, and improving ignitability;
(ii) Mid-injections during the intake stroke and early compression
stroke to create a homogeneous mixture of different charge
temperature or quantity;
(iii) Late injections during the late compression stroke for
charge/thermal stratification.
7.1Early injections during the NVO period R Standing & et al.[ 5 ] Investigated the effects of injection timing and valve timings on
CAI operation in a multi-cylinder DI gasoline engine having
displacement volume of 1.6 liters and a compression ratio of 11.5. A
high pressure swirl injector mounted below and between the two
intake valves was operated at an injection pressure of 100 bars. Both
intake and exhaust camshafts were equipped with VVT devices and
their cam lobes were machined to produce maximum valve lifts
around 2 mm. fig13 shows the effect of fuel injection timings on the
main combustion process. It can be seen that the earlier the fuel
injection took place during the negative valve overlap period, the
more advanced the start of main combustion when the engine
operated with a lean fuel air mixture. However, in the case of
stoichiometric mixture shown in fig 14 fuel injection at 20 BTDCduring the recompression phase led to the earliest combustion,
followed by 40 BTDC and TDC injections. Delayed injection into the
intake stroke resulted in the most retarded combustion and the lowest
peak pressure in both stoichiometric and lean mixtures. In addition it
can be seen that the early injections into lean burned mixtures led to
more advanced combustion than for stoichiometric mixtures.
In order to understand better the underlying mechanisms, detailed
analysis has been carried out at Brunel University [6] on the physical
and chemical processes taking place within the cylinder by means of
3-D full cycle engine simulation. The simulation programme is based
on the KIVA3v with improvements in turbulence, the gas/wall heat
transfer, spray atomization, ignition and combustion and it takes intoaccount the gas exchange processes that are crucial to the residual gas
trapping method. The Shell ignition model was chosen and has been
modified to simulate the auto-ignition process in low temperature
combustion. For the high temperature combustion, a characteristic
time combustion model is used. The transition from auto-ignition to
the main combustion process is based on the local cell temperature:
when the temperature of a cell exceeds 1080K, high temperature
combustion model is activated for such a cell. The simulation
programme was validated against engine experiments before it was
applied to study CAI combustions.
The main combustion characteristics for injections during
the NVO period are summarised in Table 1. The value of net IMEP is
closely related to combustion phasing and pumping loss. Both too
early combustion phasing and higher pumping losses contribute to
lower IMEP values with injections at 40 and 20 ATDCoverlap, as
compared with the injection at 75 ATDCoverlap. Comparing the
cases with injections at 75 ATDCoverlap and TDCoverlap, the
combustion phasings of those two injection cases are quite similar,
however the higher pumping losses result in lower IMEP with
injection at TDCoverlap
7.2 Mid and late injections during the intake and compression
strokes Using the same 3-D full cycle engine simulation, the effect
and underlying mechanism of mid and late injections on CAI
combustion were examined Figure 15 shows the pressure and heat
release rate varying with injection timings. Comparing the two
injections during the intake stroke, the start of combustion is slightly
retarded with later injection timing(SOI at 150 ATDC Overlap), leading
to lower peak pressure. Table.2 shows that there is little difference in
the compression temperature between the two injections during the
intake stroke. The delayed start of combustion with later injection is
therefore likely related to the time available for fuel to mix with air
and subsequent low temperature chemical reactions. However, the
combustion phasing is advanced, as the injection is retarded further
into the compression stroke (SOI at 218 ATDC Overlap). This is more
likely due to the in-cylinder mixture stratification.
Based on the above studies, the mechanisms of combustion
phasing control by injection timing in a lean-burn CAI DI gasoline
engine can be summarized herewith. The factors include the
thermal/chemical effects caused by early injection during the NVO
period, or charge cooling effect by injection during the intake stroke,
or fuel stratification effect by late injection at the compression stroke.
Heat release or thermal effect associated with injection during the
NVO period has a dominant effect on advancing the start of maincombustion. The chemical effect is secondary and its presence
promotes the first stage of ignition during the compression stroke.
However, injection during the negative valve overlap period can also
slow down the main combustion process, if the in-cylinder
temperature during the recompression process is reduced
significantly due to chargecoolingeffect and hence less or no heat
release reactions can take place during the recompression and re-
expansion. The late injection during the compression stroke can lead
to an advanced combustion due to charge stratification, whilst the
injection during the intake stroke slows down the start of main
combustion by charge cooling effects.
Fig 13 In cylinder pressure and heat release with various injection
timing for lean mixtures
Fig14 In cylinder pressure and heat release with various injection
timing for stoichiometric mixtures
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Table 1 Effect of early fuel injection timing on CAI combustion at
1500 rpm & = 1.2
Fig 15 Pressure & heat rate profiles with mid & late injections
Table 2 Effect of mid & late fuel injection timing on CAI combustion
at 1500 rpm & = 1.2
CONCLUSION: CAI combustion control has been realized in
both single / multi-cylinder production type PFI and DI gasoline
engines using the NVO approach with VVA technology that allows
CPS and cam phasing. It is achieved by trapping copious amount of
burned gases in the cylinder through EVC early. Such engines are
characterized with superior fuel economy due to the lack of throttling
loss and extremely low NOx emissions. However, the range of CAI
operation needs to be significantly extended not only to the high load
region but also the low load region to take advantage the fuel
economy and low emission benefits across the vehicle driving modes.
DI is one such technology which offers more independent control
over CAI output and combustion phasing for optimized performance
and emissions as compared to PFI. It has shown that fuel injection
timing in DI can be used as an effective means to control CAI
combustion.
REFERENCES
1. Homogeneous Charge Compression Ignition(HCCI) Technology ,
A Report by Office of Transportation Technologies ,U.S.
Department of Energy Efficiency and Renewable Energy, 2001.
2. Li, J., Zhao, H., and Ladommatos, N., Research and development
of controlled auto-ignition (CAI) combustion in a four-stroke multi-
cylinder gasoline engine, SAE paper 2001-01-3608, 2001.3. Leach, B., Zhao, H., Li, Y., Ma, T., Control of CAI combustionthrough injection timing in a GDI engine with an air-assited injector,SAE Paper 2005-01-0134, 2005.
4. Li, Y., Zhao H., Bruzos N., Ma T., and Leach B., Effect ofInjection Timing on Mixture and CAI Combustion in a GDI Engine
with an Air-Assisted Injector, SAE Paper 2006-01-0206, SAESpecial Publication SP-2005, 2006.
5. Standing, R., Kalian, N., Ma, T., Zhao, H., Effects of injectiontiming and valve timings on CAI operation in a multi-cylinder DI
gasoline engine, SAE paper 2005- 01-0132, 2005.
6. Cao, L, Zhao, H., Jiang. X, Kalin, N., Investigation into the Effect
of Injection Timing on Stoichiometric and Lean CAI operations in a
4-Stroke GDI Engine, SAE Paper 2006-01-0417, 2006.7. Hua Zhao HCCI and CAI engines for the automotive industry
1st ed., Woodhead Publishing Limited ,ISBN 978-1-84569-128-8,2007.
Authors brief bio-data and photograph
Shri Santosh B. Trimbake presently Assistant professor in
Mechanical Engg Dept of College of Military Engineering Pune
411031 affiliated to JNU, New Delhi. Author is M Tech (Thermal &
Fluids Engg) from IIT Powai. Area of interest is I C Engines &
Refrigeration & Air Conditioning. He Possesses total experience of
14 years, out of which 5 years served in Defence Industry (Ordnance
factory) & remaining 9 years in teaching
*santoshtrimbake@yahoo.co.in
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