The practical side of mobile electrification

Hydraulic drives are built to handle rugged operating conditions.

Manufacturers of construction equipment are falling over each other in the scramble to introduceelectrified off-road vehicles that eliminate diesel enginesand, in some cases, hydraulic drives. A number of suchmachines were on display at this year’s bauma, andpredicting more of the same at Conexpo 2020 will be thesafest bet in Las Vegas.

Although limiting greenhouse gas emissions and combating climate change are laudable goals, OEMs need to step back and consider the engineering practicalities and underlying economics before jumping head first into electrification. In terms of power density, performance and price, in most applications electric drives absolutely won’t match hydraulics in the next few years.

Take the case of hydrostatic wheel drives. While granted, they’re less efficient than their electric or mechanical counterparts, they have a lot going for them in mobile settings — starting with compact size, high torque capacity, rugged construction, relatively low cost and proven durability.

There’s lots of talk of electric replacements for hydraulic drives. Unfortunately, systems where conventional electric motors generate the same torque are much larger, and space is at a premium in a mobile machine. One proposed solution is ultrahigh-speed electric motors that operate at around 20,000 rpm. This makes the units smaller and lighter. But then engineers need to integrate a complex planetary gearbox into the motor to reduce output speed and connect to standard drive-train components. Moreover, a traction motor typically requires an inverter that takes up space.

This motor/gearbox system faces a difficult development path to ensure it’s protected against vibration, shock, corrosion and temperature extremes, and operates reliably for a long time. And worse, high-speed permanent-magnet electric motors and the accompanying gearboxes are expensive, and inverters aren’t cheap, either.

One engineer at a major component supplier related that OEMs are beating down their door for help with electrification, for the most part to satisfy marketing hype, not to solve practical applications. “In terms of dollars and cents, it’s nuts.” Thus, over the near term the main focus of electrification will be for sites and cities that mandate local zero emissions – today mainly in Europe. Then OEMs have to get on board despite that, in most cases, the machines will cost substantially more yet provide laggard performance.

Like it or not, conventional diesel engines and hydraulic systems will continue to be used in mobile equipment for a long time because they offer a robust, reliable and well-known option served by a wide network of suppliers. And fluid-power technology, thanks to innovations like digital control, continues to improve in terms of efficiency and power.

That said, a number of factors could upset this status quo, including:

• A substantial and worldwide tightening of air pollution regulations

• Punitive taxes on carbon emissions

• Long-term spike in the price of oil

• Serious innovations that upgrade electric-drive performance and slash costs

• Breakthroughs in advanced battery chemistries

Such changes would open the door to major opportunities for electric powertrains. Over the long term fluid-power suppliers need to be attuned to potential market shifts, as it’s risky to be unprepared for the adoption of new technology.

Ken Korane • Contributing Editor