Here’s an inside look at the concepts, engineering and testing behind Caterpillar’s highly successful 336EH hydraulic hybrid excavator.
By Aleksandar Egelja, Engineering Manager, Advanced Hydraulic Systems, Caterpillar Inc.
Recently, I had the pleasure of addressing Fluid Power World’s inaugural Technology Conference in Milwaukee. The discussion centered on Caterpillar’s innovative 336EH hydraulic-hybrid excavator, and how this novel design increases fuel efficiency by more than 25%. Based on this talk, here’s a behind-the-scenes look at the project from initial concept to product introduction, and some lessons learned along the way.
Think back to 2008. Just like car owners, mobile-equipment operators were feeling pain at the fuel pump. High diesel prices were squeezing operating margins for construction equipment fleets, and they needed more-efficient machines. That, in a nutshell, was the motivation behind the development and launch of the 336EH excavator.
But the project was about more than just cutting fuel costs. Before we started the engineering process, we surveyed our customers to fully understand all their requirements. Yes, companies were looking to improve their overall ownership and operating costs by reducing fuel consumption and carbon footprint, but not at the expense of lower performance, quality and reliability.
The trade-offs were important when it comes to fuel efficiency. The simple solution is to reduce engine power, but many customers measure productivity by the tons of material per hour they produce and move. Dialing down the engine power hurts productivity and does not improve overall efficiency. We had to ensure that any new product at least matched the performance and quality of non-hybrid models while reducing operating costs.
Before we dive into the engineering details, let’s first define a “hybrid” as a machine designed to collect, store and reuse energy as it operates. That differs from machines with electric drives that do not store energy.
Many machines have the opportunity for energy storage and reuse. For instance, wheel loaders and excavators dig, lift and dump material in a truck, and then lower the bucket for the next work cycle. Lifting creates future opportunity for capturing potential energy that typically converts to waste heat (by throttling hydraulic flow) when the bucket lowers. Also, engines are sized to deliver peak power, for example, when an excavator must lift its maximum rated load. But most machines do not operate continuously at peak power.
This creates an opportunity to store energy, release it later and minimize demand from the engine. This was the basis for hybrid technology in a new excavator design: how to smartly capture and reuse power—and substantially reduce fuel consumption. Another option would be to downsize the engine and use stored energy for peak demands, when that downsized engine is not capable of delivering sufficient power.
Hydraulic versus electric
Based on customer input, in 2009 we began concepts for both electric and hydraulic hybrid solutions, concentrating on recovering energy from the excavator swing movements. It’s no secret that in recent years, some of our competitors have introduced electric-swing hybrid excavators. We considered both technologies, and took a dual-path approach that let engineers fully understand the pros and cons of both options.
We started development of electric and hydraulic versions in parallel. However, an electric hybrid eliminates the hydraulic motor and circuit that controls excavator swing motion, and replaces it with electric motors, generators, ultracapacitors to store energy, a cooling system for the power electronics and many other components.
While our customers wanted fuel efficiency, they also wanted a system that’s easy to understand and service. In comparison to electric high-voltage technology, accumulators are simple, proven devices for hydraulic energy storage. A hybrid excavator also had to be commercially viable and profitable for our customers. Our research showed system costs, and the potential savings and payback time, weighed in favor of the hydraulic hybrid.
After much study and intense discussions regarding fuel efficiency, performance and cost of both technologies, we decided to move ahead with the hydraulic hybrid using our standard 336 excavator as a baseline machine.
Here is a bit more insight into how the hybrid system works. Excavators routinely dig material and dump it in a truck. Each cycle the machine raises the load, swings the upper structure to the truck, stops, and finally dumps the load. To brake the pivot motion, conventional excavators use crossover circuits on swing motors to throttle flow and route it to tank, which generates heat. In contrast, the hydraulic-hybrid system uses accumulators to store swing braking energy and release it to accelerate in the opposite direction. High-production excavators that work continuously provide an excellent opportunity to capture and reuse energy.
To design a system that provides the needed performance and fuel savings, Cat engineers faced an intensive development timeline centered around three building-block technologies, called conserve, optimize and reuse.
Conserve essentially entails improving fuel efficiency. Based on brake-specific fuel consumption (a measure of fuel efficiency in IC engines) and engine maps, we understood it was important to lower engine speed and operate at torque values that provide the best efficiency. To get the required performance with a lower engine speed, we paired the engine with a larger-displacement pump: the Cat electronic standardized programmable pump. The pump and electrohydraulic controls ensure smooth transitions between accumulator and engine-driven pump flow, conserving energy and fuel.
Optimize can be considered the brain of the hybrid system. Its Adaptive Control System and independent metering valve (IMV) improve performance by intelligently managing flow to control motion and deliver power precisely where it is needed. The IMV replaces traditional single-spool valves on the 336 and significantly reduces fuel consumption. Being electronically controlled, the IMV operates in different modes for various conditions—such as for resistive, over-running or regeneration—and optimizes machine performance for each.
Reuse is the final technology block. Two piston-type hydraulic accumulators charged with nitrogen mount in the rear of the 336EH. An energy-recovery (ER) valve connects the hydraulic swing motor and accumulators. When the machine swings from side to side, the accumulators collect the available hydraulic braking energy, and the ER valve subsequently releases pressurized fluid when needed to power the motor.
Integrating these three subsystems, while challenging, led to significant efficiency improvements; integrating the overall hybrid system with the power-management systems of the new Tier 4 engine were key to improving overall performance.
Thus, deep integration of the components, and the algorithms developed to control them, were essential for this technology to succeed. We are big proponents of virtual product development. The first step was to create control logic and dynamic models that work together in the virtual space to predict and also refine machine performance.
For instance, the reuse system delivers energy back to the swing system, and managing pump flow versus accumulator output guarantees adequate cycle times while saving energy. Electronic controls seamlessly transition the circuit to ramp up the pump when the accumulators have discharged, regardless of the speed and load requirements of a particular application. Thus, we had to develop highly sophisticated control algorithms so the hydraulics maintains performance and efficiency, and also ensures that these actions are completely transparent to the operator.
Our engineers created software models, predicted and validated performance and efficiency and, if we didn’t achieve our goals, then further algorithm tuning was performed. As the design progressed, engineers gained a deeper understanding of different types of operating cycles to better optimize the software—before ever building a machine.
Next, we prototyped a machine with new hardware and software to validate the models’ predictions. Machine testing gave us a better understanding of the differences between virtual and actual machine dynamics, and let us refine algorithms based on fuel efficiency, controllability and performance.
Productivity, efficiency and controllability generally involve trade-offs: improving one could have negative implications on others. In this case, however, we maintained or improved productivity while dramatically increasing efficiency. At the same time, the controls proved to be solid and robust so operators didn’t experience a difference in performance between new and old machines.
Nonetheless, when you adopt new technology, people naturally worry, “Is it going to work?” Fuel efficiency was one of the key deliverables, and we applied a rigorous evaluation process to measure and verify results. To do that, we used our standard 336E and hydraulic-hybrid 336EH machines in side-by-side comparisons.
The first tests were run at the Caterpillar’s proving grounds, using our own operators to perform operations like trenching, digging, loading and leveling. Fuel efficiency improved anywhere between 20 to 48% with the 336EH, depending on the application. Fuel efficiency was tracked in terms of tons per liter, so we measured both the amount of material moved as well as the liters of fuel burned in doing so. We also stressed gathering statistically significant data, thanks to tightly controlled measurements of payload and fuel consumption, to ensure accurate results.
Next, the machines moved to various customer sites to track real-world performance. Here, the customers’ operators worked the machines with baseline and hybrid technology. Again, we measured performance parameters and generated a substantial amount of statistically significant data. It showed productivity improved 7%, fuel consumption was reduced by 27%, and overall fuel efficiency in tons per liter improved about 46%, which was quite significant in the eyes of our customers.
Given the excellent results, we began production of the 336EH hydraulic hybrid in 2012 and formally introduced it in April of 2013 at the Bauma exposition in Munich.
We now have hundreds in the field. They’re versatile machines used in different regions, with different operators, performing many different jobs in varied soil conditions. Regardless, they deliver the sizeable fuel savings our customers expect.
But validation did not stop there. We no longer track how any specific machine is used. But we have telematics capability that tells us, on a daily basis, how each machine performs in terms of fuel consumption, work time versus idle time, and so on.
So, we compared EH machines against the significant population of standard 336E excavators in the field, compiling a sample size of 22,000 machine-months of data on the two. Results show the 336EH has an average fuel savings of about 25%, regardless of the application. This is an excellent use of Big Data, if you will, in that our data analytics adds confidence that the machines provide significant fuel savings to our customers.
As this project shows, we approach hybrid technology with an open mind. If electric hybrids or some other type of hybrid solution someday surpasses hydraulics in terms of component costs, power density, reliability and other key areas, then we may consider that technology in future machines.
Looking forward: The 336EH is well-aligned with Caterpillar’s future-technology vision that revolves around “Smart Iron.” At last April’s Bauma, our CEO Doug Oberhelman explained the importance of taking our machines, that already work well, and making them more intelligent by marrying Big Data and digital technology. That furthers our customers’ goals of improving productivity, efficiency, reliability and safety. From a hydraulics perspective, we need to continually develop components, systems and algorithms to support this vision.
While most people are familiar with hybrid automobiles and know that they use a gas engine and a storage battery, I expect that few people know how construction equipment hybrids work.