Advanced combustion system

Advanced Combustion Systems

The Curses of the Engine

High Load Limitations

To avoid knock in spark-ignition (SI) engines

• SI engines must limit bore and compression ratio, which reduces thermal

efficiency and limits the available spark advance window to prevent knock; these

constraints restrict achievable power density and efficiency at high load.

• At low load operating at stoichiometric lambda (λ = 1) for emissions (three-

way catalyst operation) forces air-charge control (throttling), which increases

pumping losses and reduces indicated efficiency (ηi).

Diesel/Compression-ignition (CI) engine differences at high load

• CI engines do not suffer from knock in the same way, enabling higher

compression ratios and larger bore, and they can operate largely unthrottled

because ignition is controlled by fuel reactivity (min λ ≈ 1.3).

• However, CI engines at high load produce significant soot and NOx

emissions and generate high combustion noise due to rough combustion.

Low Temperature Combustion (LTC) Concepts

LTC Objectives and Effects

• LTC aims to achieve a well-mixed, homogeneous charge across the

combustion chamber and a homogeneous lowered temperature to suppress NOx

and soot formation.

• LTC employs high dilution via low equivalence ratio (ɸ) and/or high exhaust

gas recirculation (EGR) to reduce local temperatures and concentrations of reactive

species.

Operational characteristics

• LTC modes occupy an efficiency zone where local conditions are leaner and

cooler than conventional direct-injection CI (CDC) or SI combustion; this can

increase efficiency but makes reliable ignition harder for traditional timing-control

ignition (TCI) devices.

• LTCs need combustion behavior more simliar to CI mechanisms (CDC-like)

with fast combustion starting near top dead center (TDC) to approximate an ideal

Otto-like cycle.

Control and variability issues

• Highly diluted CI operations face significant cycle-to-cycle and cylinder-to-

cylinder variations, making consistent combustion phasing and stability challenging

LTC must provide very fast combustion with ignition timing near TDC to

Therefore

minimize variability and preserve efficiency.

LTCs are crossover modes between SI and CI.

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LTC Modes Overview

• HCCI (Homogeneous Charge Compression Ignition), PPCI (Partially

Premixed Compression Ignition), RCCI (Reactivity Controlled Compression

Ignition), SACI (Spark-Assisted Compression Ignition) and conventional CI/SI.

• Theoretical advantages include very high efficiencies (up to ~50%), ultra-low

NOx/soot, fuel flexibility. While disadvantages include combustion control/phasing

difficulty, limited load range, and engine safety constraints.

HCCI (Homogeneous Charge Compression Ignition)

Mode description

• HCCI uses a premixed homogeneous lean charge (λ up to ~8) that auto-

ignites by compression (high compression ratios reported up to CR ≈ 28:1),

producing very fast combustion with ultra-low NOx and soot emissions thanks to the

low T and the λ=8.

• Ignition occurs spontaneously across multiple spots as pressure and

temperature rise, so direct control of ignition timing is inherently limited.

HCCI Case Study: Nissan Quasi-HCCI MK Disel fuel

• Baseline geometry: bore × stroke = 85 × 86 mm, displacement 488 cc, CR =

18:1, swirl ratio ≈ 3, toroidal open combustion chamber.

• Achieved NOx reduction (~90%) and higher efficiency by using high EGR to

reduce in-cylinder temperatures and delaying injection timing after top dead center

(ATDC) to retard ignition, plus increasing swirl ratio to enhance mixing.

Development changes and outcomes

• To control premature ignition (before the EOI) and improve premixing,

changes included lowering CR from 18:1 to 16:1, increasing swirl from ~5 to ~9,

increasing injection pressure, and adding EGR cooling. Even with modifications



expanding load range remained difficult due to controllability; second-generation

approaches using up to 45% EGR achieved very low NOx and soot at low load but

suffered increased low-load UHC and CO emissions due to incomplete combustion.

Fuel property comparisons

• Low-reactivity, gasoline-like fuels suit HCCI because they are easier to

premix and less likely to auto-ignite prematurely; high-reactivity diesel is harder to

premix and tends to auto-ignite too earl (before EOI).

• Strategies to expand HCCI window at low load (due to the difficulty to auto-

ignite) include negative valve overlap (NVO=closing the exhaust valve during

discharge to increase p and T), very high EGR fractions (40–70%), and elevated

intake temperatures (IAT(electrical heater) up to ~250°C) for low-load operation.

Practical shortcomings of gasoline-like HCCI

• Main limitations include lack of direct combustion control, poor repeatability

across cycles and cylinders, high-rate pre-ignition/pressure rise rate (PRR) limiting

high-load operation, and CO/HC emissions at low load due to low peak

temperatures (~1400–1800 K) causing misfires and partial combustion difficoult



combustion to control due to the absence of the spark.

PPCI (Partially Premixed Compression Ignition)

• PPCI has premixed lean charge (λ up to ~5) with locally richer zones

produced by direct injection (DI) useful for the ignition; typical compression ratios

are in the 14:1–18:1 range.

• Ignition typically occurs spontaneously after end of injection (EOI), producing

relatively fast combustion with low NOx and soot compared to conventional diesel.

Using gasoline-like fuels in PPCI

• With low-reactivity gasoline-like fuels, early injections can create a premixed

charge while a later trigger injection controls ignition timing. usually 4 injections:



premixed, pilot, pre and main.

Combustion and emission control techniques

• Intake air preheat (~150 °C) improves fuel evaporation and low-load stability.

• Low-pressure EGR reduces NOx at high load.

• Increasing inj pressure (300–1000 bar) minimizes wall wetting and soot.

• Injection pattern optimization reduces HC/CO and soot.

Case study: Delphi GCI Gen3X (Gasoline PPCI)

• Engine geometry: bore × stroke = 82 × 105 mm, disp ≈ 555 cc, CR = 17:1 

quite standard for a gasoline engine except for the CR.

• Design elements included bowl shaping and high injection pressure (~500

bar) to limit wall wetting, enabling high CR and split injections to achieve low

emissions and smooth torque delivery.

• Low-load range and cold-start are guaranteed by exhaust re-breathing (ERB)

strategies that tune valve lift, duration, timing, and turbocharger setup during



intake phase, the exhaust valve is reopened to take back exh gases T↑



• PPCI balances low NOx and soot (worse than HCCI) with good efficiency at

both high and low loads; however, it still faces control and phasing challenges and

increased complexity. Also the PRR(press raise rate) is higher than SI/CI engines

due to high CR high peak of boost pressure during the cycle reliability ↓

 

RCCI (Reactivity Controlled Compression Ignition)

• RCCI uses dual fuels with different reactivities: a direct-injected high-

reactivity fuel (HRF) triggers ignition at low load, while port-injected low-reactivity

fuel (LRF) establishes a premixed lean charge; λ up to ~4 and CR typically 14:1–

18:1.

• Ignition near TDC after EOI, giving fast combustion with low NOx and soot.

Fuel choices and split strategies

• Fuel combinations: HRF (Diesel (77%), Biodiesel, n-heptane (15%));

LRF (gasoline (43%), alcohol (25%), gas fuels (23%), and i-octane (9%)).

• Control uses HRF-LRF split percentage and injection timing similarly to PPCI,

but with the added lever of fuel reactivity split to control phasing and emissions.

RCCI Advantages, Limits, and Integration

• RCCI provides wide range efficiency and can achieve higher loads while

maintaining low NOx and soot at many operating points.

• RCCI have a bigger covering area on the load map, but the efficiency and the

BSFC (brake specific fuel consumption) are worse than PPCI.

• RCCI typically shows very good efficiency at high load and strong low-load

performance; it improves control and reduces PRR risk compared to HCCI but

introduces greater system complexity.

• Multi-modal control strategies could exploit different modes (HCCI, PPCI,

RCCI) across loads, but a fully multimode engine would be overly complex in

practice; higher loads and cold conditions may still require spark assistance.

SACI (Spark-Assisted Compression Ignition)

SACI Basics

Mode description

• SACI uses a spark plug in combination with a low-reactivity fuel (LRF) and

stratified charge to ignite a premixed lean charge and then promote multipoint

compression autoignition 2 ignitions (one from spark and other from



compression) the combustion speed up thanks to the peripheral compression



ignition spots.

• Typical compression ratios are moderate (≈12:1–14:1) to prevent knock,

resulting in fast combustion with low NOx and soot.

• SACI combustion stages include spark discharge, early kernel growth, flame

propagation-driven combustion, and multipoint autoignition-driven combustion.

• Control parameters: spark timing, injection timing and split, cooled EGR for

high-load NOx control, and NVO (negative valve overlap) at low load to increase p,T

and generate multiple CI spots.

• especially at high loads SACI has a high COV (coefficient of variation) low



repeatability of the cycle.

• SACI uses spark to extend controllability into conditions where autoignition

alone would be unreliable, enabling reduced PRR (pressure raise rate) risk and

better low-load stability compared to HCCI.

SACI Case Study: Mazda Skyactiv-X SPCCI

Hardware and calibration highlights

• The Skyactiv-X SPCCI uses electronic variable valve actuation (eVVA),

external EGR with cooler and valve, a high compr