Chrysler's new V6 continues with speed-density calibration

  • 09-Jul-2010 12:20 EDT
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Underhood photo on the 2011 Grand Cherokee shows the V6's manifold air pressure sensor (against the firewall). Chrysler continues with speed density calibration approach. (Image by Paul Weissler)

The 2011 Jeep Grand Cherokee platform may have been developed in conjunction with that of the Mercedes M-Class of former owner Daimler, but the powertrain reflects a purely Chrysler approach. The 3.6-L Pentastar V6 is a Chrysler development and is made in the company's Trenton, MI, engine plant. And its calibration carries a Chrysler signature: "virtual sensing" by speed density instead of with a mass airflow (MAF) sensor.

The speed-density algorithm does rely on sensor inputs but only those for manifold absolute pressure (MAP), intake air temperature and oxygen, engine rpm, and valve timing—the ones already in place even with a mass airflow system. Chrysler is the only Detroit-based automaker that has never used the MAF sensor.

Chrysler has been refining its speed density approach for over 20 years, touting the greater reliability. A sensor that isn't there not only doesn't incur a parts cost but also obviously can't fail or even lose accuracy from dirt accumulation or aging of the element.

However, the nearly direct reading from an MAF sensor makes calibrations easier. They're faster to do, inherently very accurate, and calibration engineers have devised algorithms to deal with any sensor drift from aging.

Further, because the final calibrations are done more quickly, that is much appreciated by chief engineers as a completed task, among the many things to do in the lead-up to Job 1.

At the same time, Chrysler has been improving its algorithm, which is based on the ideal gas law and features a modeling system for volumetric efficiency—a key metric in calculating airflow. The algorithm also requires accurate predictions of air temperature entering the combustion chamber, when the only input is the temperature sensor at the fresh air intake. In this case, too, the company's decades of experience are a key enabler.

Chrysler also has learned how to use oxygen sensor signals to adjust for variations both from one engine to another and in fuel quality, two areas where the mass airflow sensor readings would provide some guidance. Most important, Chrysler engineers say they have learned to do the speed density calibrations as quickly and accurately as calibrations with an MAF sensor.

Most of Chrysler's experience had been with fixed-camshaft engines, which are easier to calibrate with speed density than engines with variable valve timing (VVT). This is because with fixed cams the relationship between volumetric efficiency and load for a given rpm is linear, explained Mike Dahl, Head of Powertrain Controls Engineering.

However, before the new V6 was launched, Chrysler gained VVT calibration knowledge as part of the so-called World four-cylinder gasoline engine program shared with Hyundai and Mitsubishi. It resulted in developing a complex math model-based neural network, which Chrysler engineers employ to be able to fill in the tables with on-target airflow and temperature numbers.

Chrysler also employs a "stringent verification" of every calibration to ensure it also meets mass production and life-cycle requirements.

Although the speed-density algorithm may seem dated, it actually is used even with systems that have MAF sensors, as the MAP sensor responds faster. So although steady- state numbers are generated with the MAF sensor, speed density typically is used for transitions.

By comparison, General Motors long has used "Dyna Air," a combination method that was based on mass airflow for steady state and speed density for transient conditions.

In addition, speed density also is often used with mass airflow systems to check the MAF sensor rationality for On Board Diagnosis II (OBD II) regulatory requirements, Dahl noted.

With Chrysler now under Fiat management, a logical question is whether or not the company would continue with speed density as its calibration approach. A Fiat signature technology is "Multi-Air," a sophisticated electro-hydraulic variable lift/timing system that regulates intake airflow from cylinder to cylinder at every engine running stage.

Is this too complex to be compatible with speed density? Apparently not, for with Multi-Air Fiat has a system that continuously regulates airflow precisely into each cylinder, which would seem to be helpful.

And Chrysler confirms that impression, noting that the Fiat 1.4-L Multi-Air "FIRE" engine, which shortly will reach the U.S. in the Fiat 500, also uses speed density—and the approach is very similar to Chrysler's.

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