Hybrids On the Hill

Last week I went to a Hybrid Truck exhibit and briefing near Capitol Hill organized by HTUF (Hybrid Truck Users Forum). On display were about a dozen trucks for various vocations (refuse trucks, school buses, delivery vans, and a long-haul truck
tractor) featuring technologies we've read about. These included electric and hydraulic hybrids, not only for the propulsion but also for auxhiliary loads.

These make the most sense in stop-and-go applications. Every time a vehicle comes to a stop, kinetic energy is converted into another form. Traditionally vehicle brakes have done so by converting that energy into heat and dissipating it to the
atmosphere, a process that wastes a lot of energy.

What a hybrid system does, whether electrically, hydraulically, or in some other means, is capture that energy for later use.

In some of these trucks that energy can be used for propulsion as well as for PTO (Power Take-Off), such as powering the hydrualic ram in a refuse truck, power tools at a work site or the lift bucket for power crews. This work can be done using stored energy rather than idling the engine to produce the power.

Hybrid electric trucks use the same operating principles as those of most hybrid cars. They capture braking energy and convert it into electricity, which is stored in batteries (or capacitors).

On the other hand, hydraulic hybrid trucks capture braking energy via a hydraulic pump and two connected accumulators (tanks which store hydraulic fluid under pressure). One tank is a low pressure tank, the other a high pressure one. When the vehicle slows, the pump forces more fluid into the high pressure tank, increasing the stored energy for later use (see the Parallel Hydraulic Hybrid diagram).

To my knowledge, the vehicles on display were parallel hybrids and the sense was that series hybrids are on a longer time horizon [correction: the UPS parcel delivery van present was a series hybrid, thanks Eric].

The main issues with implementation, of course, are reliability and Return-On-Investment (ROI). These vehicles have yet to be deployed in large numbers so there are questions about how they'll perform in the real world, if they'll deliver the
expected benefits. Further, because of the low production volumes the costs are still considerably more expensive than conventional vehicles.

It's a classic challenge. That's why it's important for policy makers to help make the hump smaller and encourage industry to find the answers to the two questions above.

Regenerative Braking Energy

Regenerative braking is a significant part of the increased efficiency of hybrid vehicles. Out of curiosity, I did a few back-of-the-envelope calculations to figure out how much energy is involved with stopping different vehicles from different speeds.

The basic equation is 0.5mv^2 where m is the vehicle mass and v is velocity. Let’s take a look at a few different vehicles and the amount of energy that might be recouped during a stop. Most of the following calculations are done in metric units.

Assuming a 3,300 lb (1,500 kg – how convenient) sedan coming to a stop from 50 mph (22.4 m/s). That works out to 376 kilo Joules (kJ).

For a 44,000 lb (20,000 kg) vehicle such as a refuse truck making a stop from 20 mph (say 9 m/s) that’s 810 kJ.

A tractor-trailer loaded to 60,000 lb (27,270 kg) at 65 mph (29.1 m/s) 23,092.5 would need to scrub off 23,093 kJ.

Checking Wikipedia for the energy content of gasoline and diesel yields estimates of (32-34.8 MJ/L and 40.3 MJ/L respectively. As an aside, ethanol has estimated energy content of 18.4 to 21.2 MJ/L, seemingly barely half of that of diesel.

Another figure I’ve come across is from a NextEnergy brochure about hydraulic hybrid vehicles can “capture and reuse over 70% of the energy normally wasted during braking”. If you could do that for the above 3 scenarios, each of those stops would regenerate 263 kJ, 567 kJ, and 16,165 kJ respectively.

However, assuming gasoline engine efficiency of 25% and diesel engine efficiency of 35% (my SWAG) you would need 1,052 kJ (263 kJ / 0.25), 1,620 kJ (567 kJ/ 0.35), and 46,186 kJ respectively to generate that power from fuel. In other words, to get the tractor-trailer in this example moving from a stop to 65 mph takes about 1/3 gallon of diesel fuel.

All this energy is currently just wasted to the atmosphere as heat from regular disc or drum brakes to bring things to a stop. Depending on a vehicle’s typical drive cycle, there is substantial potential savings in regenerative braking. Which is why they’re starting to be used on heavy vehicles that do a lot of stop-and-go driving such as refuse trucks and delivery vans.

It comes down to balancing space requirements, added weight, cost, the volume and maximum pressure capability of the accumulator, and fuel savings. I wonder what applications they’ll appear on next.