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Making emissions an inside job: refined Mahle Monotherm piston design helps engine builders meet new standards. (Engine Technology).

In all the recent developments of emissions-compliant engines and the technologies used to meet regulations -- cooled EGR, aftertreatment devices or Caterpillar's recently unveiled ACERT system -- what many have overlooked is the role engine component suppliers have played in providing products that help engine manufacturers hit their emissions targets. While these run the gamut from cooling packages to fuel systems to electronics, one of the key aspects of emissions has been the development of refined piston designs.

A good example is the latest generation of Monotherm piston developed by Mahle for use in the 2002 emissions-compliant Cummins ISX diesel engine for on-highway truck and other applications. The Monotherm piston patent was first achieved in 1996 and prototype deliveries began in 1997. The first engines to use the design in production entered the market in 2000 at another U.S. engine manufacturer.

The introduction of the Mahle piston-equipped ISX diesel marks new business for the company and marks the second engine manufacturer to use the Monotherm pistons in the North American market. Over the next two years Mahle will introduce pistons for seven additional engines at three different customers in North America. Six additional programs are set to begin after 2004, the company said.

The Monotherm is a single-piece steel piston designed to replace the conventional two-piece articulated piston currently used in most heavy-duty applications within the U.S. As manufacturers continue to introduce engines with improved performance that are designed to meet strict emissions requirements, they are finding the Monotherm piston essential to meeting those challenges.

Compared to conventional articulated piston designs, the Monotherm design offers several advantages, according to Mahle. One is reduced piston skirt friction losses due to reduced thermal expansion of the steel skirt which leads to less variation in skirt clearance -- approximately one percent higher output, which results in 1 percent lower specific fuel consumption, the company said. Depending on the compression height, Mahle said it can achieve lower weight with the Monotherm piston design than that of the conventional articulated piston.

The Monotherm piston design allows lower compression height than the articulated or any welded design piston. A reduction in compression heights as high as 50 percent can be achieved, according to Mahle, which noted that the aluminum skirt of the articulated piston needs sufficient material and space above the pin which in turn requires a larger compression height.

This Monotherm design also allows an increase of peak cylinder pressures to 3770 psi (260 bar) and higher. "The peak cylinder pressures are going higher with the new emissions-compliant engines," said Wolfgang O. Rein, vice president product engineering at Mahle. "This is causing the need for stronger materials. There was a desire to replace the aluminum pistons with steel for durability With the Monotherm design we are able to do this."

"The closed skirt piston design came about for Tier 2 engines," added Joe Strong, director sales and application engineering for diesel engines at Mahle. "With the Monotherm design, there is less material between the top of the piston and the piston pin. The shorter pin reduces weight and gets the piston on par weight-wise with an aluminum design. Lower weight pistons also help engine manufacturers with engine balance problems.

"The EGR systems of the new Tier 2 compliant engines increase corrosion and wear concerns. However, the Monotherm pistons, with the special Mahle phosphate coating, also successfully prevent corrosion which can occur in the new EGR-equipped engines."

The steel design also increases load-carrying capacity as the piston uses a larger pin for durability The catastrophic failure mode of the new piston also results in reduced engine damage, Mahle said, as the tear-apart force of Monotherm design is much higher than articulated pistons thanks to the higher structural strength.

The Monotherm piston design can reduce cavitation of the cylinder liner, even with thin-wall liners, the company said, which allows an increase in piston diameter within existing engines. This enables engine manufacturers to use thinner cylinder liners for better cooling, a key factor in hotter-running emissionized engines.

Larger bores also offer the possibility for higher displacement and power with the same engine block. The thermal expansion of the skirt pad and cylinder liner are similar, allowing minimized installation clearance from 40 to 50 [micor]m vs. 80 to 100 [micor]m, improving piston guidance. This has also reduced engine noise, since closer tolerances yield less piston slap at low load and cold start conditions.

In addition, the one-piece design enables easier assembly and simpler logistics. Mahle has also noted a reduction of lube oil consumption and blow-by.

Mahle said its Monotherm pistons are available in production diameters ranging from 130 up to 180 mm.

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Comment:Making emissions an inside job: refined Mahle Monotherm piston design helps engine builders meet new standards. (Engine Technology).
Author:Mercer, Mike
Publication:Diesel Progress North American Edition
Geographic Code:1USA
Date:Jun 1, 2003
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