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How To Decide Which Engine To Pair With Your Big Rig Transmission

Fleet managers have a hard-enough time juggling the daily needs of a transportation company. That’s why they occasionally need a how-to on the best way to spec equipment or decide on the best approach to procurement. Recently we looked at how to spec a transmission, but there is more to the powertrain than just shifting.

Fleet managers have a hard-enough time juggling the daily needs of a transportation company. That’s why they occasionally need a how-to on the best way to spec equipment or decide on the best approach to procurement. Recently we looked at how to spec a transmission, but there is more to the powertrain than just shifting.

How To Decide Which Engine To Pair With Your Transmission

In fact, it could be argued that the engine is the single most important part of the powertrain. Without it, the truck doesn’t move, and the job does not get done. Unfortunately, there is no one-size-fits-all solution to determining what engine you should choose. There are different vocations and duty cycles to consider.

Even more, without the right engine and transmission combo, a truck might find itself overworked and fleet technicians may find themselves completing far more reactive maintenance on the vehicle than should otherwise be necessary. On the flipside, the truck could potentially be underworked – in other words, your bottom line will suffer because you spent money on equipment that you didn’t actually need.

The key is choosing the powertrain that is perfect for the job. Selecting the right combination is very important. There are a dozen or so factors that fleet procurement managers must consider when they are setting about the task of figuring out the right powertrain for the right application.

Application is Key

The first step in ensuring you get your powertrain combo right is to consider the application the equipment will be operating in. Factors that should be considered include:

  • Vehicle operating weight
  • Maximum starting grade
  • Road surface

Some transmissions have specific operating condition limitations. So, maximum grade and road conditions can help determine what torque and gearing are necessary. It is critical to consider what the truck will be hauling and how heavy the load will be. How fast will your truck drivers be driving, and will they be expecting inclement weather or other road hazards.

Specific variables like these must be taken into consideration when figuring out how much power should be sent to the wheels and what the axle ratio should be. As one example of a potential way this can be approached is to consider smaller displacement to save on weight. Once you have considered application, it is time to look at drivability.

Is it Drivable?

You may be thinking, “What is drivability?” A vehicle’s drivability refers to the smoothness of the vehicle’s operation. Is there vibration in the seat? Does the vehicle maneuver well? These may not seem like they have anything to do with the powertrain, but in fact, they do. Duty cycle and application set the direction on how a powertrain should be spec’d.

Whether it be through smoother shifting or the ability to proceed forward slowly – such as for utility applications – the powertrain should supplement these efforts, especially if there is a critical safety factor at stake. As one example, some trucks may have to operate in curb work applications. Conversely, for fire trucks, they need to be able to offer quick and smooth acceleration.

In other situations, such as high stop and start scenarios, clutch wear and truck driver fatigue must be considered. While automated manual transmissions (AMTs) have a clutch, they don’t face the same wear-and-tear problems that traditional clutch-driven manual transmissions do.

Transmission Types

While we are discussing transmissions, it is important to cover the choices related to transmission type. It is important that the application the transmission will be operating in matches the design, truck driver experience levels, the required power and torque for the application, and more.

Automation is great for just about any truck driver. Even truck drivers who are experienced with manual transmissions may enjoy the ease-of-use that automation provides. AMT transmissions help reduce truck driver fatigue and potential distraction.

In fact, a change in fleet attitudes towards transmissions has caused a major industry shift. Many fleet managers will now tell you that it is far harder to find a manual transmission than it used to be. A large percentage of fleets have already made the transition to AMT.

Examining Drive Types and Axle Ratios

After a motor carrier has decided on the transmission it will pair with their engine choice, it is important to decide on whether they want direct drive or overdrive. Overdrive varieties have historically been the go-to decision, simply because there were few direct drive manuals and a limited axle ratio, but the AMT has changed all that.

Today, fleets can easily find AMTs that offer both direct drive and overdrive varieties. And since a direct drive transmission is a bit more fuel efficient than an overdrive transmission, fleets are increasingly making the switch. Axle OEMs have also been introducing new ratios, so direct drive powertrains can still maintain their high level of efficiency without making performance sacrifices.

Speaking of axles, the rear-axle ratio must be considered when selecting a powertrain. Lower numerical axle ratios are generally selected when a vehicle has a direct drive transmission. Overdrive layouts typically come with higher numerical axle ratios. The powertrain choice bears a direct impact on cruise speed and top gear time, so a tailored solution is required to ensure you are getting the most out of your vehicle.

What is the Cost of Ownership?

While performance factors are probably the most important, another important consideration – especially for budget-minded fleet procurement managers – is the total cost of ownership they can expect in a particular powertrain combo. While purchase price is important, it is only one factor in the equation.

Responsible fleets must also consider fuel economy, maintenance, resale value, and potential repair costs. Looking at these factors could be the critical deciding factor on what equipment is right for the job. The application the vehicle will be operating in is also an important consideration where total cost of ownership is concerned.

If it has not already become apparent, application is critical to just about every factor regarding powertrain selection. Knowing the application will allow you to pay for only what you need, while ensuring performance does not suffer.

Moreover, a proper understanding of a piece of equipment’s total cost of ownership will help determine which engine/transmission combo should be purchased. It will prevent needless tradeoffs between performance, safety, and efficiency.

Nearly every fleet manager is faced with a similar problem: Increasing efficiency while decreasing overall costs. In the hustle-and-bustle of being more efficient and effective, fleet managers must not forget that machinery purchases are much like real estate purchases. Making a large purchase is all about timing, and what may seem like a good idea today may not necessarily be a good idea tomorrow.

Equipment combos that require a lot of maintenance eat into total cost of ownership. Trucks that don’t handle the duty cycle well because of bad procurement choices break down prematurely and weigh heavily on a motor carrier’s bottom line.

Performance Factors

In discussing total cost of ownership, another factor is the priority you place on performance or economy. While truck drivers enjoy operating vehicles that perform, fleet managers look at fuel economy and efficiency as their top concerns.

If fuel economy is the priority, then you may want to select a down-sped or low RPM cruise powertrain. If you have top performance on the mind, then a traditional power curve powertrain will be your best bet. Again, it is important to consider application.

If you are operating a snowplow truck, a high-numerical gear ratio will ensure that the vehicle does not suffer performance loss at low speeds, but also remains stable and startable in cold weather. Conversely, if the vehicle is operating in a situation where maximum fuel economy is preferred, a lower gear ratio is better.

The Most Important Person

In all this discussion of equipment, fuel economy, performance, and safety, it is critical that we do not forget the most important part of the truck: the truck driver. Choosing the right powertrain is vital to ensuring your truck drivers remain happy.

In fact, providing engine/transmission combinations that are easy to operate allow proper control of a vehicle and prevent the truck driver from becoming overly stressed out. The skills of those operating the vehicle should also be taken into consideration. New truck drivers may not be as apt as experienced truckers, so powertrains that don’t require as much from them should be considered.

Furthermore, with car OEMs continuing to eliminate manual transmissions from their lineups, trucking OEMs are following suit. Staying on top of the trend while also keeping truck driver comfort in mind can go a long way to ensuring you make the right choices.

The fact is, your entire operation is riding on your powertrain. The engine/transmission combo is the most important spec that goes into a truck. Get it wrong, and you may find yourself spending way more time, money, and effort on problems that could have been prevented from the outset.

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