Among mobile-equipment manufacturers, electrification — the concept of replacing diesel engines and traditional hydraulics with electric drives and batteries — is without a doubt a hot topic. But OEMs should consider both the financial and engineering ramifications before jumping in head first, according to Thomas Schulze, Drive System Application Engineer with Liebherr Components in Biberach, Germany.
“The power density of hydraulics in most applications can’t be matched by electric drives at present. That’s because it is so much greater and the cost benefit is the main factor for a lot of applications,” he said. But if government regulations demand equipment redesign, say for zero emissions, then it will make sense to have a strategy for electrification, he continued.
One obvious problem with electrification in mobile machines is how to fit in enough batteries to supply sufficient power to operate a machine for a reasonable time. “There will be some mobile applications in which batteries make sense,” said Schulze. “But when applications require a lot of power and extensive driving, a diesel-electric unit is essential.” Look at the numbers, he continued. “An installed kilowatt-hour costs around €500 and if a machine requires 70 to 80 kWh, you have €40,000 of investment costs only for a battery. It won’t work. So the main focus of electrification will be for sites and cities that mandate local zero emissions. Then we have to do it, despite the fact that today, in most cases, there is no cost benefit,” he said.
“But not only is a battery necessary for electrification. Another requirement is an electric motor with the same torque and size as a hydraulic motor. A standard electric motor would be much larger, and you just don’t have the available space in a mobile machine,” said Schulze.
“For Liebherr, the solution is high-speed electric motors. A normal electric motor may run at 1,500 up to 4,500 rpm. But if it could operate at 14,000 or 20,000 rpm, the motor gets smaller and lighter. It becomes possible to use a more compact motor design for the same drive performance,” he said.
“But then you have the problem of the 14,000 rpm input to a gearbox that normally has a maximum limit of 3,000 to 4,000 rpm.” That is overcome with a high-speed planetary gearbox stage specifically optimized in terms of cooling and lubrication. It integrates into the motor and reduces output speed to connect to today’s standard drive-train components. The gearbox technology is currently in development and Schulze expects it to be in production within the next few years.
A number of companies have approached Liebherr Components to support them in the electrification of their machines. Serious electrification efforts, however, require a detailed analysis of the machine operation and the drive system to determine what power, torque, and speed is really required, said Schulze.
With a well-scaled product series of different-size electric drive solutions, it becomes possible to realize various electrification concepts to suit a wide range of applications and torque requirements, he explained. These include:
• Grid fed via a plug-in cable, for applications with high power demand and small operation range, such as crawler excavators that only move a few meters at a time.
• Battery electric, for applications with low to medium power demand and large operation range.
• Diesel-electric hybrid, for applications with high power demand and large operation range.
To sum it up, mobile machines have to be compact and lightweight and, in most cases, that currently weighs in favor of hydraulic drives, said Schulze. The question with complete electrification is whether a breakthrough in battery technology with regard to size, weight and cost is possible within the coming years. As far as the development of electric drive solutions is concerned, offerings from the Liebherr Components Division will soon catch up with the advantages of hydraulic drives.