LO 7-3 Identify and explain the purpose and functions of major piston construction features

Mono Steel Pistons

A majority of newer diesel engines now commonly use mono steel pistons. Brand names for these from Mahle are MONOTHERM® piston, also known as Ecotherm pistons , and are the newest innovation in diesel piston design Figure 20. The most noticeable feature of the first HD engines using these pistons is the one-piece alloy steel design, which uses less material between the piston crown and the piston pin Figure 21. Shortening the piston achieves nearly a 50% reduction in the distance between the centerline of the pin bore and the top of the piston—a dimension referred to as the compression height. Thinner skirt walls, a large cooling oil passageway, and the lower compression height help make these pistons comparable in weight to aluminum pistons used in similar applications. The smaller compression height has the potential to enable the use of shorter connecting rods and reduced block height to lower engine size and weight.

FIGURE 07-20: Ecotherm pistons from Mahle are single-piece alloyed steel pistons used in MD diesel engines.

FIGURE 07-21: A MONOTHERM ® piston by Mahle is a one-piece alloyed HD steel piston design.

Mono steel pistons are also a type of LCV piston . Mono steel piston design offers the following advantages:

1. Alloyed steel, although heavier than aluminum, is stronger than aluminum. The strength of the alloyed steel allows less material to be used in the construction of the piston, resulting in an alloyed steel piston comparable in weight to an aluminum piston. The fact that mono steel pistons are short also makes them lighter.
2. Steel expands at a lesser rate than aluminum, so operating clearances are much smaller for mono steel pistons. This reduces piston noise at cold-start and low-load conditions when operating clearances are greatest. Tighter operating clearances also reduce blowby and oil consumption.
3. One-piece steel design increases load-carrying capacity. This means higher peak combustion pressures are sustainable. The current maximum cylinder pressure for mono steel pistons is 3770 psi (260 bar). This advantage alone increases power density from an engine, which means more power can be produced from each cubic inch (or cubic cm) of engine displacement.
4. Higher load-bearing capabilities enable the use of larger cylinder bores for higher displacement volume and power output within the same engine block. Originally, these pistons were used in engines with approximately 15 L (915 in 3 ) displacement. However, increasing numbers of small- and mid-bore–size engines are now using these one-piece steel pistons Figure 22.
5. Increased power output and lower fuel consumption. Fuel consumption in test conditions is as much as 2% lower. Reduction of the size of the piston skirt and the corresponding reduction in friction, plus the tighter operating clearances of steel, provides 1% higher power output and a 1% reduction in fuel consumption. This improvement also allows turbocharger speed to decrease by 1000 revolutions per minute (rpm) in some engines, which leads to improved turbocharger durability.
6. Reduced engine friction. A smaller skirt area results in reduced cylinder wear. Much of the improvement in efficiency is achieved through a reduction in friction and tighter operating clearances.
7. The strength of the alloyed steel reduces tear-apart forces during catastrophic failure; this leads to less engine destruction.
8. A one-piece design makes assembly and service simpler. No piston pin bushing is required by the steel construction. Instead, the wrist pin bore is coated with phosphate for reduced friction. A black phosphate coating over the piston crown also reduces the possibility of corrosion due to exhaust gas recirculation (EGR).

FIGURE 07-22: The use of mono steel pistons can reduce a piston’s compression height.