Smaller is Better: T4F engines under 75 hp don’t sacrifice torque, require less maintenance

Updated Nov 24, 2017

Maintenance Lead 1117 E1511375153251

With the emergence of the Tier 4 Final era, most of the big changes in diesel engines have been on those above 75 horsepower. They required new emissions reduction systems using selective catalytic reduction (SCR) and diesel exhaust fluid (DEF).

But in most cases, engines below 75 horsepower hit their air quality targets by upgrades from mechanical to common rail fuel injection and electronic control. This technology has been featured in larger engines for more than a decade. It’s proven, well-known and not nearly as expensive or as complex as SCR and DEF systems.

“Electronic control paired to the high-pressure common rail fuel system allows for extremely precise control of the fuel-injection events several times throughout the combustion cycle in all operating conditions,” says Rob Shailes, strategic business development manager for Kubota Engine America. “This methodology allows for the cleanest possible raw emissions from the engine prior to introduction into the engine aftertreatment device.”


DOCs and DPFs

Many of the smaller engines also can, as a general rule, absorb and chemically convert contaminants in the exhaust using a diesel oxidation catalyst (DOC), rather than needing the larger diesel particulate filter (DPFs). More common on engines above 75 horsepower, DPFs require regular regeneration, or “regens” as they’re sometimes called, to turn trapped soot particles into ash.

Some of the first generation (Tier 3 and Tier 4 Interim) of these big engine DPFs required periodic cleaning to remove the ash, but manufacturers have tuned the performance of the engine so that today’s DPFs last the life of the engine with no maintenance requirements and perform the regens automatically and invisibly to the operator.

DOCs are also zero-maintenance devices. But some smaller engines still use DPFs depending on the size constraints on the equipment, application requirements and how the OEM tunes other elements in the exhaust reduction system.

“It is important when a DPF is used that the engine is running at a sufficiently high duty cycle in order to effectively burn off captured particulates,” says Shailes. “A DOC has the advantage that it is a non-capture, chemical conversion device and as such is not subject to performance sensitivity associated with a low duty cycle.”

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Engines that run hard at high loads generate enough heat to maintain a DPF, while machines that run at idle or at low rpms most of the time are better suited to DOCs.

Deutz characterizes it’s less than 75-horsepower engines as “space optimized” or “performance optimized,” says Alec Hurley, at Deutz. While this typically involves a choice between DPF or DOC, the real difference is in the engine software. “Let’s be honest about it,” he says. “The technology today is not in the pistons and combustion systems. Those are important, but the software, electronics and data sets are where it’s all done today.”


The 75 hp barrier

As you might expect, the SCR systems on engines greater than 74 horsepower brought a substantial price increase, plus additional maintenance for the owner.

“For many OEMs, there is a big incentive to keep engine power below the threshold where SCR is required,” says Oliver Lythgoe, product concept marketing manager, Perkins Engines.

And that threshold is 75 horsepower.

“It is not only the cost of the DEF tank, lines, pumps, injectors and aftertreatment, but also the need for the operator to top up the second (DEF) tank,” says Lythgoe. “This can be a particular challenge in rental machines where operators may not be familiar with these technologies.”

So, it may come as a surprise when you go to replace that skid steer or backhoe you bought five or 10 years ago to find it’s no longer powered by a 90- or 100-horsepower engine, but instead runs on a 74-horsepower diesel. Should you be concerned? Must you pony up the big bucks to get the horsepower you used to enjoy?

Manufacturers say no. In fact, in the Tier 4 Final era, you’re getting a better engine and machine all around with little or no sacrifice in performance.


Less horsepower, more torque

“What you’re starting to see is the 100-horsepower machine, the premier machine, with very little compromise, giving the same performance at 75 horsepower,” says Hurley. “OEMs realized with some adjustments to their transmissions or hydraulics, they could manage it. Most operators can’t tell the difference.”

Hurley cites Deutz’s work with a rough-terrain crane manufacturer to adjust the torque curves and minimize the continuous loading of the hydraulics at certain points to achieve what he calls “near zero” changes. “Maybe the only compromise was 1 or 2 miles per hour on the top speed. That was immediately seen as totally acceptable, and as a result, there has been a huge product line coming out at 75 horsepower.”

“Multiple manufacturers including Kubota have developed 74.4-horsepower versions of their engines with a high-torque performance feature,” says Shailes. “The increased costs and application complexity associated with the 75-horsepower-plus aftertreatment led OEMs to accurately evaluate their actual power/torque requirements and led them, in many cases, to be able to use a 74.4-horsepower engine with enhanced torque characteristics. I personally know of OEMs that field tested machines with 74.4-horsepower, high-torque engines where 100-horsepower engines had been previously installed without telling the operators. Almost exclusively the operators did not notice any significant reduction in machine performance.”

Torque is what gets the work done, says Lythgoe. A 74-horsepower engine with huge amounts of torque at low speeds will be very productive. “On the Perkins Syncro 3.6, we have a special curve that delivers flat 74 horsepower all the way back to 1,200 rpm,” he says. “So, max torque would be 312 pound-feet or 192 percent of the torque at its nominal rated speed.”

Engines below 25 horsepower have changed little and generally retain their mechanical fuel injection systems. “In the same way that many OEMs dropped below the 75-horsepower threshold in order to reduce the costs brought about by mandatory aftertreatment components, OEMs are, where feasible, reducing their power requirements below 25 horsepower to use mechanical engines with no aftertreatment devices,” says Shailes.


Tier 4 Final advantages

“Fuel consumption should be noticeably better,” says Lythgoe. “And while the engine helps, machine integration is the key to the biggest fuel economy savings. That’s why so much of our focus goes into working on collaboration around machine integration.”

Noise is also reduced, Lythgoe says. “Operators can expect a more comfortable environment.”


Care and feeding

Thanks to their advanced design, Tier 4 Final engines do not require much maintenance, says Hurley, other than changing fluids and filters. “We have engines coming back with up to 15,000 hours on them that have never been opened up until we do the rebuild. In the old days, if you remember, the worst place to find an oil leak would be the valve cover because somebody had been in there adjusting valves. But hydraulic-valve actuation eliminates the need for lash adjustment.”

As with any engine using high-pressure common rail injection with electronic timing, fuel quality is important. “It makes sense to use a fuel supplier you trust. Most operators these days pay attention to their fuel supply,” says Lythgoe. “Genuine brand filters are also probably a good investment. Why risk a lost day’s work, and a $1,000 fuel-system repair for the sake of $5 on a filter?”

Hurley agrees.

“If anything, the biggest issue is still bad fuel,” he says. “You have to minimize human error. You’re not going to change that.”