MAHLE study finds its Turbulent Jet Ignition pre-chamber initiated combustion system can deliver HCCI-like fuel economy in a conventional PFI engine
|Fuel economy comparison of TJI and stoichiometric SI combustion in same test engine. Source: W. Attard. Click to enlarge.|
MAHLE Powertrain’s Turbulent Jet Ignition (TJI) spark-initiated pre-chamber combustion system (earlier post) can achieve HCCI-like fuel economy across a drive cycle using a conventional PFI (port fuel injected) engine with much less complicated hardware and a much simpler control system than HCCI, according to a detailed analysis presented by MAHLE at SAE 2011 World Congress.
Results of this study showed that the pre-chamber jet ignition combustion system roughly matches HCCI fuel economy across both NEDC and FTP drive cycles, with a 13% improvement in fuel economy recorded over the baseline stoichiometric (λ=1) spark ignition combustion system. The system also reduces engine-out NOx by more than 99%.
...the jet ignition combustion system utilized a production base PFI platform with a relatively low compression ratio of 10.4. In optimized form, a near 25% drive cycle fuel economy improvement was predicted with the jet ignition combustion, achievable with the addition of side direct injection, increased compression ratio to take advantage of the knock limit extension and pre-chamber jet-optimization.—Attard et al.
|Engine-out NOx emission comparison of TJI and stoichiometric SI combustion in same test engine. Source: W. Attard. Click to enlarge.|
William Attard and his colleagues note in their paper that these results would exceed the drive cycle fuel economy improvement achieved with other low temperature combustion technologies such as HCCI in the same engine platform, as there is no requirement to switch back to conventional spark ignition combustion at high and low loads. The switching of combustion modes limits fuel economy improvements and maximum compression ratio for knock avoidance.
Background. Efforts on combustion initiation in pre-chamber cavities in spark ignition engines reaches back to the 2-stroke Ricardo Dolphin engine in the first part of the twentieth century. Since then, many variants have been developed. 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. MAHLE’s TJI system is characterized by auxiliary pre-chamber fueling; small orifices connecting the main and pre-chamber combustion cavities and a very small pre-chamber volume.
The smaller orifice size causes the burning mixture to travel quickly through the orifice, which extinguishes the flame and seeds the main chamber with partially combusted pre-chamber products (radical species). It is then the reacting pre-chamber combustion products which entrain and ignite the main chamber charge through chemical, thermal and turbulence effects some distance away from the pre-chamber, thus producing a distributed ignition system. In addition, the smaller orifice creates a turbulent jet that penetrates deeper into the main charge. To avoid impinging on the combustion chamber wall, the pre-chamber volume has to be kept relatively small.
Benefits of small pre-chamber volumes compared to their large counterparts include negligible power loss and fewer hydrocarbon (HC) emissions due to the reduce crevice volume and combustion surface area.—Attard et al.
|Computer-generated image of the Turbulent Jet Igniter, designed to replace the spark plug in a modern spark ignition combustion system. Click to enlarge.|
MAHLE’s TJI can utilize the original PFI (homogeneous) or DI (homogeneous or stratified) fuel system supplying the main combustion chamber, and the majority of the fuel can be carried over from the standard OEM engine design. Besides the main chamber fueling, a small amount of additional fuel is injected into the pre-chamber just before ignition in the region of the spark plug.
Earlier studies by MAHLE show that the TJI system, which replaces the spark plug, has supported peak indicated net thermal efficiency of 42%. Additionally, the pre-chamber combustion system is capable of tolerating up to 54% mass fraction diluent (excess air and EGR) at the world wide mapping point, resulting in near zero engine-out NOx.
The new study. The research presented at this World Congress was designed to:
Present the current speed load range of TJI operation.
Compare performance, efficiency and emissions over an extended speed load map (mini-map) for the TJI system, operating with varying excess air dilution.
Analyze combustion burn parameters and pressure rise rates over the mini-map.
Compare fuel consumption benefits of various combustion modes in the same engine platform, including stoichiometric spark ignition, homogeneous lean burn spark ignition, HCCI and lean burn jet ignition over NEDC and FTP driving cycles.
Based on current experimental data, estimate the potential drive cycle fuel economy improvements achievable with Turbulent Jet Ignition combustion technology in an optimized engine platform.
Thee research engine was derived from a production level I4 PFI Ecotec LE% (2.4 liter) GM engine. The 0.6-liter single-cylinder research engine featured a flat top piston crown in a modern pent roof combustion system which incorporated double overhead camshafts and four valves per cylinder. Continuously variable camshaft phasers for both the intake and exhaust enabled variable valve timing. Standard production intake and exhaust profiles were retained.
Results. Fuel economy improvements across the test matrix ranged from 10-20%, with efficiency improvements attributed to a combination of combustion improvements, the near elimination of dissociation due to the low combustion temperatures and significantly educed engine throttling.
Reductions in pumping losses were evident, with the test engine being completely de-throttled at approximately half load (~4.7 bar IMEP), corresponding to an exhaust lambda in excess of two.
MAHLE said that the high excess air dilution tolerated with this combustion system is caused by the very fast burn rates as the ignition system is able to produce multiple, widely distributed ignition sites which consume the main charge rapidly. The resultant low temperature results in the near elimination of engine out NOx.
...strong synergies exist when coupling jet assisted ignition with engine downsizing at both high and low engine loads. One way forward for light load involves applying the ignition system to an already heavily downsized engine and operating homogeneously ultra lean, then switching to a conventional near stoichiometric fuelling strategy at higher loads. This could negate the need for lean NOx aftertreatment and could be one method forward for future part load fuel economy improvements as the downsizing limits are approached and swept capacity can no longer be reduced due to turbocharger, abnormal combustion or engine mechanical limitations at elevated BMEP.
Strong synergies also exist at high load when applying the jet ignition combustion system to a heavily downsized platform. The controllable faster burn rates allow for a base compression ratio increase or further swept capacity reductions when compared to conventional spark ignition combustion due to the knock limit extension associated with the distributed ignition system. This is achievable as the increased flame propagation (reduced flame front travel path) associated with a distributed ignition system reduces the likelihood of end-gas knock due to the reduced residence time. Hence, the potential fuel economy advantages of applying the jet ignition combustion system to downsized powertrains are clearly evident.—Attard et al.
William P. Attard, Michael Bassett, Patrick Parsons and Hugh Blaxill (2011) A new combustion system achieving high drive cycle fuel economy improvements in a modern vehicle powertrain (SAE 2011-01-0664)