Thermal management isn't a new frontier for improved fuel economy, but automakers are finding that seemingly small changes add up and that system integration provides opportunities to do more with less.
The 2013 Ram 1500 pickup HFE (high fuel efficiency) uses a host of effective ways to get class-leading truck fuel economy, and the U.S EPA window sticker numbers are impressive:18 mpg city, 25 mpg highway for the 3.6-L V6 with idle-stop and eight-speed automatic—a 20% improvement over the previous model. The standard V6/eight-speed model (without idle-stop) is close: 17 mpg city/25 mpg highway.
The eight-speed automatic accounts for six of the 20 percentage points, and although the 3.6-L V6 and low-rolling-resistance tires also contribute, thermal management plays an important part, too. The previous generation had a 3.7-L V6 with only the six-speed automatic and EPA numbers of 14 mpg city/20 mpg highway.
At the front are computer-controlled "active" grille shutters, a considerable advance from their historical origin: a manual "winterfront," primarily on trucks and buses, to restrict grille airflow to raise coolant temperatures for cabin heating. Next we began to see computer-controlled shutters on premium European cars as a way (with a road speed input) to improve vehicle aerodynamics. The Ram 1500 pickup is a step ahead.
If forward motion ("ram") airflow is more than sufficient for the front-end cooling module (condenser, radiator, and transmission oil cooler), the shutters close to the extent possible and excess airflow runs over and around the front end. The Ram's coefficient of drag is 0.360 in the HFE vs. 0.376 for the standard model and 0.386 for the 2012 edition. A truck bed tonneau cover also is used on the HFE, and it's a measurable contributor to the HFE aero, too.
Most road-speed-triggered active shutters close for improved aero at 35-40 mph (56-64 km/h) if coolant temperatures are in an acceptable range. The Ram's more comprehensive strategy, however, uses a test-proven map that is based on coolant, engine oil, and transmission oil temperatures and road speed (the shutters may be closed until as low as 10 mph/16 km/h, and they open at about 80 mph/129 km/h and above, to pre-empt engine cooling). The map logic also incorporates radiator fan operation and air-conditioning system high-side pressure to provide even greater range of control.
The traditional thermal or electronically controlled clutch fan on trucks is replaced on the Ram by radiator electric fans that are pulse-width-modulated (PWM). They operate at specific duty cycles for more precise fan speed control, rather than just on or off, or at a choice of fixed speeds. PWM fans are used on some cars and crossovers, but the Ram adds a dc-to-dc converter to the electrical system to ensure uniform voltage is always available for all devices, also including lighting, audio, and HVAC.
The Ram features an active transmission warm-up system with an oil-coolant heat exchanger on the transmission. The hoses are connected to the engine cooling system. Although this is one of the items eligible for the EPA-proposed off-cycle CAFE carbon credits (corporate average fuel economy), Chrysler integration engineer Jamie Standring said it was in the original engineering plan for the Ram program to provide best-in-class fuel economy.
The engine cooling circuit has both a three-way valve for coolant and a bypass valve in the transmission oil cooling circuit so the transmission oil can be warmed up more quickly from a cold start, which reduces parasitic loss from the drag of the spinning gears. If the transmission oil is getting too hot, such as during towing, the bypass valve setup directs oil from the heat exchanger and into the transmission cooler in the front-end cooling module.
The system originally also included an electrically heated engine coolant thermostat controlled by the powertrain module. It was intended to warm up the engine oil faster by circulating warming coolant through an oil-to-coolant heat exchanger for another reduction in parasitic losses from the crankshaft churning through the oil. However, during final preproduction durability testing, the thermostat wiring connector failed. The continuing test results showed no difference in fuel economy, which was attributed to the fact that a low-viscosity oil was being used (5W-20) and the grille shutters were closed, producing approximately the same effect on coolant (and engine oil) warming. This showed how important the shutters were in thermal management. In this case, that they would permit taking the electrically heated thermostat out of the picture. An oil of even lower viscosity (0W-20) also is possible if improvement could be validated, Standring added.
This discovery could permit Chrysler to get two off-cycle carbon credits for the grille shutters—one for active aero, another for the active engine warm-up from the shutters' strategy (along with the low-viscosity oil) that results in faster oil warm-up for reduced drag as the crank spins. Because they're extra credits (that is, off the EPA drive cycles), they will apply only to CAFE, not to window sticker. The schedule of credits still is going through the EPA regulatory development process; when the credits become final, they will apply retroactively and could be carried forward. There is no specific hardware requirement, so if a manufacturer accomplishes the objectives with special strategies and existing components, and shows good data to EPA, credits will be awarded.
The credits are applied only according to the percentage of qualifying production. The EPA-specified fleet average is 250 g/mi, but each company is assigned an exact target, based on the types of vehicles it produces. Trucks may receive higher off-cycle credits than cars. Active engine oil and transmission oil warm-up is worth 1.8 g/mi each for cars and trucks, but idle-stop for trucks would provide 4.5 g/mi vs. 2.9 g/mi for cars.
The Ram HFE is Chrysler's first vehicle with an idle-stop system, and it also was integrated—including shutters operation and idle-stop A/C-off blower speed control—into the thermal management system. Chrysler declined to provide the control strategies.
The active warm-up and idle-stop also have a positive effect on the city cycle window sticker results, with the Ram gaining 1.7% from the warm-up system and about 3.3% (equal to 1 mpg) from idle-stop. But these window sticker values are understated for the real world, because there are just two cold starts and few full stops in the EPA cycles. The off-cycle credits are intended to compensate for the five-cycle shortfalls, as well as accomplish some environmental objectives.
Electric power steering, not normally thought of as a thermal management factor, does eliminate the power steering cooler. That reduces the front-end airflow required, and although the amount may be small, the shutters' operating strategy reflects the validated overall airflow need, so for aero it is a positive contribution.
(AEI-captured video of Mike Cairns, Head of Ram Truck Engineering, explaining weight-reduction measures and other key changes to the 2013 model, including an air suspension system adapted from the Jeep Grand Cherokee, can be viewed on the SAE Video site, or on YouTube at http://youtu.be/xb1Wie5KpY4.)