Efficiency gains are attributed to a combination of combustion improvements, reduced heat losses and the near elimination of dissociation associated with low combustion temperatures. Peak efficiency improvements equate to an 11% relative improvement when compared to conventional stoichiometric spark ignition combustion.

Results also indicate that jet ignition combustion has the potential to exceed 45% indicated net thermal efficiency (19% relative improvement) with a CR increase to ~14. The CR increase is made possible by the burn rate improvement associated with the distributed ignition system and the addition of side, wall guided DI. This would exceed the HCCI peak thermal efficiency of 43% in the same engine platform as there is no requirement to switch back to conventional spark ignition combustion at high load operation, which limits HCCI maximum CR for knock avoidance.

—Attard and Parsons, 2010-01-2196

MAHLE’s TJI research builds on previous work in the field that can trace its origins back to combustion initiation in pre-chamber cavities in the first part of the twentieth century with the 2-stroke Ricardo Dolphin engine. Since the Dolphin, many variants of the pre-chamber combustion system have been developed—including the Honda CVCC in the 1970s—primarily to increase the engine’s dilution tolerance and thermal efficiency. Pre-chamber combustion systems currently exist in large bore stationary engines used for power generation, but have failed to penetrate the vehicle market.

Jet ignition is one type of pre-chamber combustion system, in which hot gas jets produced by the pre-chamber system are introduced into the cylinder where they rapidly induce ignition of the main in-cylinder mixture.

A major advantage of jet ignition systems is that they enable very fast burn rates due to the ignition system producing multiple, distributed ignition sites, which consume the main charge rapidly and with minimal combustion variability. The locally distributed ignition sites allow for increased levels of dilution (lean burn/ EGR) when compared to conventional spark ignition combustion. Dilution levels are comparable to those reported in recent homogeneous charge compression ignition (HCCI) systems.

In addition, jet ignition systems have the potential for combustion phasing control and hence speed/load range benefits when compared to HCCI, without the need for SI-HCCI combustion mode switching. The faster burn rates also allow for a base compression ratio increase (1-2 points) when compared to spark ignition and when combined with diluted mixture combustion, provide increased engine efficiency.

—Toulson et al., SAE 2010-01-2263

Computer-generated image section view highlighting the Turbulent Jet Igniter centrally installed in the test engine’s four valve, pent roof combustion system. Click to enlarge.

MAHLE set out to make its jet ignition system more commercially feasible by applying off-the-shelf hardware and modern control systems to a novel pre-chamber design. Additionally, the system has been developed to operate on readily available commercial fuels (e.g. gasoline, propane, natural gas), overcoming the hurdle of earlier jet ignition systems of using hydrogen as the pre-chamber fuel. The system has been developed as a simple bolt-on addition for a modern engine design.

With TJI, the original PFI (homogenous) or DI (homogenous or stratified) fuel system supplying the main combustion chamber and majority of the fuel is carried over from the original OEM engine design. Besides the main chamber fueling, under typical operating conditions, a small amount of additional fuel is injected into the pre-chamber just before ignition in the region of the spark plug.

Timeline of TJI sequence events over one complete engine cycle. Click to enlarge.

By locating the spark plug in a small pre-chamber (approximately 2% of the clearance volume), heat losses can be minimized and HC and CO emissions can be controlled, MAHLE said. Pre-chamber fueling event is timed to end at approximately 50° before the spark discharge, thereby ensuring that a rich plentiful mixture can be contained in the pre-chamber. Use of the rich pre-chamber mixture leads to the production of radical species which have major benefits in chemically enhancing combustion.

Novel features of the TJI system include:

• Very small pre-chamber volume (~2% of the clearance volume). This minimizes crevice volume, HC emissions, heat loss, surface-to-volume ratio effects and pre-chamber residual gas.

• Pre-chamber connected to main chamber by one or more small orifices (~1.25 mm diameter). This promotes flame quenching and penetration into the main chamber. The reacting pre-chamber mixture initiates main chamber combustion in multiple locations through chemical, thermal and turbulent effects.

• Separately fuelled pre-chamber (flush-mounted electronically controlled direct injector), allowing a rich mixture to be contained in the prechamber while the main chamber is heavily diluted (excess air and/or EGR). Injector location assists in scavenging the pre-chamber residuals and minimizing crevice volume.

• Separately fueled main-chamber (electronically controlled PFI or DI). This allows homogeneous or stratified main chamber mixtures (HC/NO_x emission control).

• Spark discharge initiated pre-chamber combustion (flush-mounted electronically controlled spark plug), allowing simple combustion phasing control.

• Research targets using readily available commercial fuels for both main and pre-chambers, enabling the use of existing refueling infrastructure.

The engine used in the experiments was derived from a current production level Ecotec LE5 GM engine. The addition of continuously variable camshaft phasers for both the intake and exhaust enabled variable valve timing. The spark ignition experiments were completed with regulated gasoline which was port fuel injected at production fuel pressures (4 bar). Jet ignition tests were completed with a mixture of gasoline and when required, a very small amount of propane when running excess air dilution.

In addition to the efficiency and NO_x results, other findings were:

• Turbulent Jet Ignition combustion is capable of matching the load operating range of conventional spark ignition combustion. This is achieved with equivalent peak BMEP and the ability to operate in an unthrottled mode down to 3.9 bar IMEPn with increasing dilution levels (excess air and high internal residuals). This minimum unthrottled load corresponds to an exhaust lambda of 2.25, which is a 50% increase in the dilution relative to the spark ignition lean limit of lambda 1.5.

• The pre-chamber combustion system is quite robust and largely unaffected by changes such as variations in spark plug type, orientation, location and electrode gap for the spark plug, as long as combustion inside the pre-chamber can be initiated.

The MAHLE team also proposed a vehicle implementation strategy for Turbulent Jet Ignition combustion, which involves switching from ultra-lean burn to stoichiometric conditions in order to achieve engine load control and subsequent emission clean-up.

The ultra-high efficiencies achievable with Turbulent Jet Ignition and the simplicity of the mechanical hardware and control system offer unique opportunities of coupling this technology to hybrid/range-extender vehicle applications.

—Attard and Parsons, SAE 2010-01-2196

Resources

• William P. Attard and Patrick Parsons (2010) A Normally Aspirated Spark Initiated Combustion System Capable of High Load, High Efficiency and Near Zero NO_x Emissions in a Modern Vehicle Powertrain (SAE 2010-01-2196)

• William P. Attard and Patrick Parsons (2010) Flame Kernel Development for a Spark Initiated Pre-Chamber Combustion System Capable of High Load, High Efficiency and Near Zero NO_x Emissions (SAE 2010-01-2260)

• Elisa Toulson, Harold J. Schock and William P. Attard (2010) A Review of Pre-Chamber Initiated Jet Ignition Combustion Systems (SAE 2010-01-2263)