There’s a simple precept in designing a truck for fuel efficiency: use as little fuel as possible to keep the vehicle moving. The laws of physics, however, dictate that a certain quantity of energy is needed to accomplish a certain amount of work.
The laws of physics haven’t changed, but the load on the engine has. With tires that require less energy to roll and aerodynamics that minimize wind resistance, today’s engines do not need to work as hard as they once did to maintain road speed.
Volvo Trucks now took that principle to heart when it introduced downspeeding. Running the engine at a lower RPM means the pistons travel up and down fewer times to cover the same ground. Last year Volvo Trucks went even further, adding turbo compounding which allows the engine to deliver the same horsepower but with less fuel.
Continuously running at high engine RPM is counterproductive to efficiency because you burn more fuel and you have more internal friction.
Director of Product Marketing at Volvo Trucks
Volvo was the first in the industry to introduce the downspeeding concept in 2011, dropping engine RPM at cruise speed from about 1,350 to just over 1,150. That 200-RPM drop reduces fuel consumption by about 1.5 percent per 100 RPM, or about three percent in total. Today’s GHG 2017-compliant D13TC engine uses turbo compounding to further reduce engine speed and fuel consumption with no decrease in performance.”
Turbo compounding converts energy normally wasted in the exhaust stream into usable horsepower at the crankshaft. This means that less fuel has to pass through the injectors to deliver the same energy output to the wheels.
Volvo Trucks designed its unique D13 with Turbo Compounding in-house and specifically for the new VNL. It starts with a simple turbine wheel located downstream of the actual turbocharger used to manage engine intake air. Exhaust gases spin this turbine which is linked through a network of reduction gears to the engine’s crankshaft. The net effect is adding additional torque and horsepower to the crankshaft.
Unlike turbo compounding used in the past, Volvo decided not to use the technology to add horsepower; Volvo takes advantage of the otherwise wasted exhaust energy to reduce fueling demand to achieve a specific torque and horsepower rating.
“Think of it this way: I’m getting some extra power from the turbo compound unit, what do I do with it?” asks John Bartel, Volvo Trucks’ chief engineer in powertrain. “I can either add it to my max power and deliver more power, or I can reduce how much fuel I’m putting in the engine for the same output. We choose the latter.”
By combining turbo compounding with a downspeeding driveline, Volvo Trucks is able to further reduce engine RPM at cruise speed, increasing the fuel economy sweet-spot. This reduces engine speed at cruise by an additional 100 RPM and allows the transmission to stay in top gear longer in rolling terrain, netting a fuel economy gain of about 3 percent.
Typically, for every gallon of diesel burned, only about 42 percent of the energy content actually makes it to the wheels
Product Marketing Manager at Volvo Trucks
“The rest is wasted as heat. We are using this system to increase that percentage. It will increase the percentage of BTUs used from every gallon of diesel. In doing so, we are actually putting less fuel through the injectors and that is where we save 6.5 percent compared to the current non-turbo-compound D13 engine.”
Downspeeding has been around since the nineteen-eighties, and turbo compounding was used on heavy aircraft engines during the second world war. Despite that, a successful pairing of downspeeding and turbo compounding could not have happened 10 years ago. The missing link was the automated manual transmission – that Volvo introduced to the industry as the I-Shift.
What was needed was not just an efficient engine, but an integrated powertrain that worked in harmony to yield the most fuel efficient operating state possible.
“By using integrated drivelines with the I-Shift, we can control it better,” says Moore. “We know the shift points and we can match those to the peak horsepower and torque of the engine curve. And in doing so, we make the proper shifts at the right time and make it efficient.”
It would be very difficult if not impossible for even the best driver to maintain the concentration needed to optimize every gear shift or change in throttle position to meet the varying terrain.
Lowering the engine’s peak torque to 900 RPM and the optimal engine speed at cruise to 950 does not leave much margin for error. It’s fuel efficiency technology at the edge of the envelope. Turbo compounding has enabled extreme downspeeding while the D13TC and I-Shift work together to precisely mate engine speed to road conditions. Sometimes there’s just a hair’s breadth difference between dropping a gear or not, but this powertrain can consistently maintain that margin in a way a driver with a manual transmission could not even hope to match.
Unlike turbo compounding used in the past for strictly performance, Volvo Trucks is the first manufacturer to be able to incorporate turbo compounding for fuel efficiency and performance benefits. The D13TC is a simple and robust system that requires no additional maintenance. In the new VNL, customers will see significant savings in operating costs as well as operational efficiencies. The D13TC is as much at home on an Interstate as on a two-lane highway thanks to the broad peak-torque range. Volvo Trucks continues to drive progress in innovative technologies aimed at providing customers fuel-efficient solutions.
It's truly a unique engine
on the new D13TC
The XE packages and the new D13 TC engine is a perfect match. The turbo unit compresses the heated air to turn a flywheel connected to a gear, which returns an additional 50 hp to the drive train. The D13 TC maintains peak torque while cruising as low as 1050 rpm, saving fuel without sacrificing power.
fuel efficiency improvement
generate peak torque as low as 900 RPM
with a 2.47 Rear Axle Ratio, you can generate peak torque while cruising at 1050 RPM
ideal operating speed
Coast to coast