Little engines that can: Big power from small displacements
Posted: February 2, 2018 by Jim Park
The Cummins X12.
TORONTO, ON — Canada has always been big-bore territory: big trucks, big loads, big hills, big engines. Right? If you’re one of those with feet firmly planted in the big bore camp, it might be time to re-think that position. Some of today’s smaller engines are surprisingly capable. They are lighter and more fuel-efficient, and deliver performance that’s nearly equal to their larger brothers.
Don’t think for a minute that we’d try to convince anyone pulling Super-Bs around British Columbia that an 11-liter power plant is going to do the job. It won’t. But in less-demanding applications, there just might be more wiggle room than you’d think. We know of many fleets — really big, successful fleets — that are happily using 13-liter engines to pull B-trains, though most of them are based somewhere east of Banff, Alberta.
In Canada, 13- and 15-litre engines are the mainstay. With the exception of the aforementioned B-trains, you can almost split the group down the middle, with fleets hauling tridems, quads, and larger loads opting for the bigger blocks, and those hauling U.S. weights siding with the mid-size engines.
All of the 13-liter engines, by our research, now hit or exceed 500 horsepower (Paccar’s MX-13 is advertised at 505), with torque has high as 1,850 lb-ft (Mack’s MP8 tops the list at 1,860). That was 15-liter territory not that long ago. Cummins’ X15 tops the 15-liter group at 605 horsepower and 2,050 lb-ft. The extra couple of liters get you an additional 100 horsepower and 150 additional pound-feet of torque.
Looking at the 11-liter group, 425 horsepower is typical, with the MX-11 juiced to 430. Torque-wise, these engines are doing 1,550 to 1,650, which is pretty respectable.
And then there’s the soon-to-be-released 12-liter X12 engine from Cummins. It sits nicely between the 11- and 13-liter models, but blows the 11-liter models out of the water with a 500-horsepower vocational offering, and 455 horsepower for on-highway applications. It offers torque of 1,700 lb-ft.
“We have targeted the X12 for weight-sensitive applications,” says Cummins’ on-highway marketing communications manager, Russ Poling. “We’re introducing a new platform with a unique displacement that offers customers the best of both worlds: light weight and high performance.”
While most engine makers won’t share their market breakdowns by displacement, Brian Daniels, Detroit powertrain and component product manager, offered that 15-liter engines, such as the Detroit DD15, make up approximately 65% of his Class 8 business, while 13-liter engines such as the DD13, account for the remaining 35% of that segment.
“The DD13s are targeted to more weight-sensitive applications and shorter BBC applications,” he says, “The DD15 has the advantage of longer maintenance intervals, a greater B50 design life, along with a resale value premium compared to smaller-displacement engines. The majority of our truckload customers prefer 15-liter engines. Regional and vocational applications often prefer 13-liter.”
Navistar’s heavy duty product segment marketing director, Jim Nachtman, told Today’s Trucking that the vast majority of his LT-model on-highway tractors sold in the U.S. are shipped with 450-horsepower engines.
“The sweet-spot rating seems to be 450-horsepower for our on-highway customer,” he says. “That gives them the power and performance they are looking for.”
Since none of the current 11-liter offerings quite get to 450 horsepower, that would seem to leave them standing at the door, but Volvo Trucks powertrain marketing manager John Moore begs to differ.
“Our typical on-highway customer is still buying the D13, but we are making inroads with the D11,” he says. “It’s easier to convince a weight-sensitive customer that the D11 is a good engine on the highway, but we’re selling more of them now to truckload carriers that don’t historically run heavy, say 65,000 to 70,000 pounds gross weight.”
At 425/1,550, it can certainly do the job. In fact, Moore says the D11 is good for up to 110,000 pounds gross weight, and up to 125,000 with engineering approval (depending on the application).
The difference between 11 and 13
All the producers of 11-litre engines claim there’s a slight improvement in fuel efficiency with the smaller engines, owing mostly to friction reductions with the physically smaller moving parts, and some improvement in air and fluid handling.
“It’s a pretty small difference,” says Paccar vice president and Peterbilt chief engineer Landon Sproull. “We see up to a 2% difference in fuel consumption between the 13- and 11-litre engines. Most of that comes from reduced friction from the relatively smaller moving parts — the pistons, the crank bearings, the generally lighter physical mass.”
Volvo’s Moore also claims combustion is more efficient in the smaller engine.
“Smaller displacement engines will create more load in the cylinder for the same output than a comparable bigger block engine will,” he says. “The more a cylinder is loaded [pressure], the more efficient it is. There are also reductions in internal pumping losses.”
The transmission sees no difference between the two — 400 horsepower is 400 horsepower regardless of the engine size. It’s the same with torque.
What the rest of the truck does with that torque and horsepower can also make a difference. Take downspeeding for example.
“In comparing 13-liter and 15-liter engines, size does not in itself dictate efficiency,” says Daniels. “This is why our downspeeding approach is so important when it comes to operating at peak efficiency. As we have seen in the market since the introduction of Automated Manual Transmissions, we are moving engines down in speed to where they are most efficient.”
There are opportunities for downspeeding in Canada, especially with lighter loads, but heavier loads require additional horsepower. That pushes the sweet-spot engine cruising speed higher and out of the range most would call “downsped”. Cruising at 1,400 rpm, for example, rather than 1,600 (where terrain allows), is still a couple of hundred fewer rpms than we once needed to maintain 100 kilometers per hour with a tridem load.
“The amount of fuel it takes to move a specific vehicle over a given route will be determined by the engine’s efficiency, which is a matter of the engine design,” says Daniels. “Another factor will be the powertrain specification that allows the engine to operate at its best efficiency point.”
The powertrain spec’ could be an on-highway downspeeding spec’ with really tall rear ends, or a city pickup and delivery spec’ where the rear ends are optimized for start/stop operation and climbing out of below-grade loading docks. Either way, fleets can get into trouble when trying to improve utilization by running the truck in something like a pickup and delivery operation during the day and a linehaul application at night. Depending on the spec’, one application or the other is going to suffer.
Don’t spec’ for the 5%
On the other hand, fleets with the breadth of equipment can spec’ engines and powertrains optimized for certain applications, such as 11-litre engines for pickup and delivery, 13-liter engines for U.S. loads and tridem-weight linehaul service, or a 15-liter engine for the heavier hauls and mountainous terrain.
“Where you can spec’ the truck for specific applications, you have the opportunity to really drill down to what works the best,” says Moore. “Where you can get into trouble is expanding the range of the operation to where it’s outside the truck’s design envelope.
“That said, if you’re only going to spend about 5% of your time outside that design envelope, don’t worry about it,” Moore adds.
There is always some overlap when it comes to determining the best engine choice for different applications and duty cycles. Weight, performance, and fuel economy all matter now, and customers certainly have a lot to choose from. While it would be easy to pigeon-hole the 11-liter blocks to local or vocational work, they are quite capable in linehaul applications up to 80,000-pound Gross Vehicle Weights.
“In addition to improved fuel efficiency strictly from the displacement, these smaller engines have lower reciprocating mass and reduced parasitic losses, which add to the efficiency gains,” says Scott Barraclough, Mack Trucks’ technology product manager. “Canada is unique with many applications with higher [Gross Combination Weights] than we see in the U.S. With that said, our MP7 – which is available with up to 425 horsepower – fits well in applications where that power level is sufficient, especially those which are weight-sensitive.”
There is at least one engine in this class claiming a B10 life of 1.6 million kilometers, and two engines claiming a B50 life of 1.9 million kilometers, which should inspire some confidence in their durability.
Some of the 13-litre engines are performing well in B-train service, and International told us of a customer in upstate New York that is hauling Long Combination Vehicles across the Thruway at 140,000-pound Gross Combination Weights with its new 12.4-litre A26 engine.
“The customer is happy with the performance of the engine in that application and they have more on order,” says Nachtman. “I think when you start talking about lines in the sand, where you need this engine or that one for a specific application, it comes down to the customers’ requirements. Then it becomes a conversation between the customer and our engineering department.”
As a final note, Canadian fleets may have to get used to using smaller engines if Canada’s version of Phase 2 Greenhouse Gas reduction regulations copy the U.S. version verbatim, as is currently expected.
“Canada will see those regulations sometime in the spring, and the consumer base may have to change what they are buying,” says Sproull. “The currently proposed GEM [Greenhouse Gas Emissions Model] favors the 13-liter engine, and OEMs have to maintain a neutral credit balance. The 15-liter performance engines are really detrimental to that credit balance. I can see where OEMs will be promoting smaller engines for the benefits they will get from the GEM model.”
Small block pros and cons
In the most general terms, here are some of the factors working for and against small block engines.
smaller engines with lower ratings will work harder, increasing exhaust temperatures and possibly minimizing aftertreatment maintenance
bigger-block engines tend to have better resale value due to the perception that they are more durable
physically smaller engines have some packaging advantages that allow for easier underhood maintenance and more aerodynamic frontal profiles
smaller engines are lighter and often less expensive upfront than a engine with a similar rating but larger displacement
a scan of current product offerings reveals that maintenance intervals for the 11-, 13- and 15-liter engines are very similar within specific brands