Mahle gasoline-only pre-chamber jet ignition combustion system yields diesel-like thermal efficiency of 42.8%; 45% achievable with engine upgrade
Mahle Powertrain has refined its pre-chamber turbulent jet ignition combustion system (earlier post) to function robustly using only gasoline. Vaporized gasoline successfully substitutes for the pre-chamber propane used earlier over all comparable operating conditions.
With this gasoline-only system, Mahle has shown a diesel-like peak net thermal efficiency of 42.8% (190 g/kWh ISFCn) and single digit engine-out NOx using a single-cylinder research engine derived from an I4 port fuel injection (PFI) GM Ecotec (LE5) 2.4-liter production unit. This could deliver more than a 20% peak fuel economy improvement when compared to stoichiometric spark ignition, according to Mahle.
A thermal efficiency of 45% would be achievable with a base engine hardware upgrade (such as side direct injection and a higher compression ratio), according to Dr. William Attard, Head of Engine Research at Mahle Powertrain, during a talk at the SAE 2012 High Efficiency IC Engine Symposium. This could result in about a 25% drive cycle improvement in fuel economy, he said.
Attard and Hugh Blaxhill from Mahle also published a more detailed paper on the technology in conjunction with the SAE 2012 World Congress.
|Jet Ignition assembly simply replaces the spark plug in a contemporary engine design. Click to enlarge.|
In such a pre-chamber combustion system, combustion in the main chamber is initiated by jets of partially combusted pre-chamber products which provide a high-energy ignition source. (The concept of pre-chamber combustion has been around for almost 100 years, and has been realized in a variety of approaches and products, starting with the 2-stroke Ricardo Dolphin engine.)
The resulting widely distributed ignition sites allow relatively small flame travel distances—enabling short combustion duration and high burn rates. Benefits of this include ultra-lean operation (λ>2) at part-load and high-load knock improvement near stoichiometric conditions.
Pre-chamber combustion is essentially a highly dilute combustion system...The control strategy is very, very simple which is one of the main advantages of this combustion system. Essentially it is exactly the same as a spark ignition system, except you have the additional pre-chamber fuel injection event—and that’s timed to occur about 50 to 30 degrees before the spark discharge to allow sufficient mixing but most importantly, avoid any fuel escaping from the pre-chamber into the main chamber which will then form NOx emissions.—Dr. William Attard
Although previous versions of Mahle’s system showed promising results, it’s major challenge had been the need for a dual-fuel system: liquid gasoline for the main chamber and small fractions of gaseous propane in the pre-chamber.
The study also presented first experimental results of high-speed jet ignition combustion with the gasoline-fueled pre-chamber at unthrottled stoichiometric conditions up to 5500 rpm. The authors noted that this is the highest published speed for this type of jet ignition system, highlighting that the system can operate at comparable speeds to conventional SI combustion in the same engine. Among the other findings of the study:
A maximum of 13.2 bar IMEPn was recorded at 3500-4000 rpm with combustion stability less that 0.9% CoV (coefficient of variance, with lower figures indicating higher efficiency and more complete combustion).
Results cross the entire speed range indicated that the jet ignition system could operate at optimal maximum brake torque (MBT) spark timing without any knock mitigation strategies when using the test gasoline fuel. The knock limit extension is due to the high burn rates which consume the main charge rapidly before the end gas has time to auto-ignite, they suggested.
Pressure rise rates could be tuned to very low levels (1.5 bar/deg) using conventional calibration techniques, with retarded combustion phasing being the most successful at the aded cost of performance and efficiency.
They proposed increased dilution as a potential technique in reduced maximum pressure rise rates to enable acceptable ~20 bar BMEP operation at low engine speeds.
Spark-Assisted Jet Ignition. As part of the study validating the gasoline-only system, Attard and Blaxhill proposed a new concept—Spark-Assisted Jet Ignition—to enable stoichiometric operation without dilution at boosted high load. They described two possible configurations of the jet igniter coupled with a conventional spark plug.
With this concept, some of the main chamber charge is consumed first using the conventional spark ignition flame propagation process, with the remaining charge consumed by the jet ignition process after some time delay. This in turn increases the burn duration and thus reduces the heat release when simply compared to burning the entire charge with jet ignition combustion.
The system would require varying spark timing between both ignition sources (spark plug and jet igniter) and the jet igniter would have to be fired before the spark ignition flame front consumed the charge in the jet igniter region to ensure adequate pre-chamber combustion.
Depending on the location of the spark plug for the conventional spark ignition combustion, significant knock limit extension would also be seen with this concept due [to] the effet of the distributed ignition system associated with jet ignition combustion.—Attard and Blaxhill
William Allard and Hugh Blaxhill (2012) A Gasoline-Fueled Pre-Chamber Jet Ignition Combustion System at Unthrottled Conditions (SAE 2012-01-0386)