To achieve a given steady-state horsepower, is it better to go high-displacement or high-boost? 4

[LMF5000]

I'm comparing two different contemporary approaches to engine design in high-power farm tractors.

One school of thought is that "there's no replacement for displacement" - so we have the Cat C18 ACERT Tier III engine in the Challenger-MT800 tractor:
585 bhp, turbocharged, 6-cylinder-in line with a displacement of 18.1 liters. [brochure]

The opposing school of thought is to use a smaller engine and more boost - so we have the FPT cursor 13 engine in the Case IH Quadtrac 580:
589 bhp, two-stage-turbocharged, 6-cylinder-in-line with a displacement of 12.9 liters [brochure - bottom of page]

Both of these engines are designed for similar applications (tractors, generators, marine etc.) where they run for long periods at a significant percentage of full rated power. However, despite making almost the same power, the FPT engine achieves it with 40% less displacement (presumably by adding a second turbo).

Which of these is the better approach in the real world? I'm leaving the term "better" vaguely defined because I appreciate that there might be tradeoffs with either choice.

 

[BrianPetersen]

These are diesel engines. Whichever one gives the best BSFC at operating points representative of real world operation (which is probably at something less than max rated power all the time).

 

[CWB1]

Define "better." Cost? Fuel economy? Service life? Maintenance? Practicality?

You can approximate fuel economy from a BSFC curve, service life and maintenance/repair from BMEP, and compare both back to cost to see if differences are worthwhile for a given set of assumptions. The first thing I would recommend doing however would be to look at the lug curve and actually determine if both engines/tractors are useful for your specific application. Peak power doesn't tell you dink, its good for selling lawnmowers and pickemup trucks but as is easily seen with both, it doesn't tell you how the vehicle/machine operates at part-throttle. My new pickemup truck has almost 400 hp but is a dog compared to the old one with 320 because it lacks torque. My new 15 hp lawnmower seems like a vast improvement over the 40 year old 10 hp model it replaced, until you recognize that unlike the old, the new model is built for constant high-rpm operation, lacks the torque to operate at part throttle, guzzles significantly more fuel, is comparatively loud, and obviously wont last 20 years, nvm 40 like the old one did.

 

[Lou Scannon]

All other things being equal, a more highly boosted, smaller displacement engine will get better bsfc, as long as the concept is not taken to an extreme. Advantages of a larger displacement engine at the same rating will be better load acceptance or startability, if you like. Longevity may be better as well, since average normalized mechanical and thermal loads will typically be less.
Furthermore, the trend in high horsepower off-highway engines is strongly toward higher power density via higher boost, as the net cost per kW is more strongly driven by engine displacement than air system sophistication.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[RodRico]

I believe efficiency is the primary advantage of using boost. At a minimum, a turbo recovers wasted energy resulting from under-expansion of combustion gasses and uses it to offset pumping losses to improve volumetric efficiency. The modern application of variable boost has even greater benefits; it is essentially a means of variable displacement that allows the engine to run at full power under varying loads, and engines running at full load always have higher efficiency than those running at reduced load. Given ever tightening emissions requirements, manufacturers are forced to emphasize emissions across their fleets, so these engines are common in modern cars. I, however, prefer large non-turbo engines. They're more reliable and are much more responsive to throttle inputs, factors I value more than efficiency alone. The legendary 3.5 liter V-6 in my 2017 Honda Accord Touring Edition is near ideal for my preferences. I was bummed to hear the option has been dropped in subsequent years which offer only a turbo four.

 

[dgallup]

If "better" is total cost of operation, one would need to know the maintenance schedule and cost of the consumables and labor. These can vary widely as some may take more gallons of oil and multiple expensive filters or longer service intervals. I would also look at how long each has been in production and how much new technology each uses. While both are well respected manufactures, everybody delivers a turkey once in awhile. It's really impossible to know from a spec sheet how well engineered all the internal components are. High boost engines are especially likely to have problems with pistons cracking around the wrist pin bosses or con rod failures. These will probably only occur in a very small percentage of the product but you'll hear about them on the internet forums after a few years in the field. I also have no idea how much exhaust gas after treatment these off road engines have these days. Early generations of diesel particulate filters and catalysts have been pretty problematical in other applications.

Personally, I lean toward the lower boost engines as being more forgiving of minor flaws in manufacturing or metallurgy.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[LMF5000]

Having driven a car with a 1.4 liter naturally aspirated engine for 5 years and then one with a 1.4 liter turbocharged engine for 6 years, I much prefer the turbocharged engine. Small NA petrol engines tend to have very low torque at low revs - I had to rev it to at least 3000rpm to start feeling some force. This means a lot of exercise with the gear lever, as cruising was done at 1500rpm but overtaking necessitated going down a gear or two. My current car has a very small turbo, which is an excellent for the road. Full torque starts at 1750rpm and ends at 4500rpm (when the little turbo can't keep up with the engine any more). It has turbo lag of course, but that just means that when I first push the pedal I feel what a naturally aspirated engine can do, and 1-2 seconds after I get the full benefit of the turbo. The broad torque curve brought about by the turbo means that overtaking can normally be done without downshifting. Another nice thing about this being petrol is that the redline is 6,800rpm; Diesels have peakier torque curves but shifting must be done at unnaturally low revs for maximum acceleration as the torque declines precipitously past 4,500rpm.

To get back to my original question, I cited the two engines as an example. I'm interested in the general consensus of the advantages and disadvantages of each. I would assume that all other things being equal (including output power), smaller-displacement engines would be physically lighter and have smaller parasitic losses, are physically smaller (hence less fluid capacity and thus lower servicing costs), and seem to have better BSFC. However I would assume that generating the same torque (assuming same power at same rpm range) with smaller pistons means higher BMEP, meaning more stresses and therefore less durability, and with materials and systems operating closer to their limits, more chance of things going wrong. Also the additional boost pressure means more highly-stressed turbomachinery at the intake side (hence more initial expense and possibly more frequent maintenance). Whether one set of factors outweighs the others is something I have too little experience to know, hence this thread.

I come across this choice all the time in many power applications. If you needed a portable 1kW generator, would you get the 100cc one from one manufacturer or the 150cc one from their competitor? Or outboard motors - if you needed a 15hp outboard, would you go for the 550cc one or the 670 cc one? It seems, especially in America at least, that larger displacement is considered a selling point. The Cat engine I listed in my first post actually boasts in its marketing material about being the largest-displacement engine in its class (which is factually true). Based on this thread however, I still haven't come across a compelling reason why displacement is a positive thing to have (pun not intended).

 

[CWB1]

For solid state and other applications where the engine will live at full power most of its life, I would lean toward the smaller, higher BMEP engine. For mobile applications where throttle response and torque are critical there is simply no replacement for displacement.

 

[NormPeterson]

I'm not sure how well turbo vs NA spark-ignition engine experience correlates with compression-ignition behavior when the differences are in displacement and presumably amounts of boost.

I can tell you that my own experience with NA vs turbocharging at the 2.5L displacement level clearly favored the NA engine under any condition where the turbocharged engine would be caught off boost. Trying to use 3rd gear at 2000 rpm or a bit less in and around housing neighborhoods instead of running up closer to 3000 rpm in 2nd being a common situation, where the peak numbers (think 160-ish NA vs 260-ish turbo here - for both HP and ft*lbs torque) had little meaning.

You need entire torque curves more than you need peak power numbers, along with a decent idea about where your operating rpm range is going to fall.


Norm

 

[PackardV8]

The "displacement versus boost" question was long-ago moot as the future was obvious more than forty years ago. It took some time, but the industry has decided to go with ever smaller, ever higher-boosted engines.

FWIW, SAAB was correct in 1978. They tried to tell the industry and the buying public the future was turbocharged four-cylinder engines. All the car magazines said, "SAAB is making wonderful performance luxury cars, but no way will the public buy anything less than a V6 or V8." Today, BMW, M-B, Audi, Lexus, Infiniti, Acura, Mustang, Camaro, et al, all have turbo fours as their base engine.

jack vines

 

[NormPeterson]

Moot only as long as significant boost can be maintained, and that going from part throttle and a small amount of boost to more throttle and more boost does not entail a disagreeable amount of lag. Those issues probably aren't as significant for LMF5000's farm tractors as they tend to be in passenger cars and other light road vehicles that experience a much wider range of power demand.

It's not as much of a problem if all you care about is full throttle operation in a straight line and the associated bragging rights, either.

But I think you should look into what the GM autocross team was doing a year or so ago with the turbo-4 Camaros in order to get around the lag issue in that activity. Hint - turbocharger durability took a huge hit from what they did to keep the turbocharger wheels spinning at sufficiently high rpms.

Any entity in the business can be the first to post a specific preference as being the direction that they feel the industry as a whole needs to take. But for every SAAB - a niche mfr at best - I'm sure that there's at least one Felix Wankel.


Norm

 

[RodRico]

Does anyone have any knowledge or experience regarding hybrid electric turbochargers versus standard? These turbos do not connect the exhaust impeller directly to the intake impeller. Instead, they use the exhaust impeller to drive a generator feeding the battery and a motor driving the intake impeller. It seems like a great idea to me; they should reduce lag and simplify variable boost.

 

[CWB1]

Hybrid turbos are a rather pointless novelty IMHO. Modern automatic transmissions and electronic controls have effectively eliminated lag so long as the calibration is good and the driver demanding power. I'm a big fan of truly manual transmissions however they were left by the wayside in performance cars years ago for this reason.

 

[RVAmeche]

I think the hybrid turbo makes sense for Formula 1 cars right now because they're able to regenerate batteries with braking and then use that energy to pre-spool the turbo. For consumer cars that don't typically see a ton of extreme braking I doubt the hybrid turbo would be significantly useful.

The split turbo design that Mercedes pioneered is also very cool; they said it results in a significant decrease in air temperatures by getting the compressor away from the exhaust turbine, even with the heat of compression being input.

 

[Lou Scannon]

they said it results in a significant decrease in air temperatures by getting the compressor away from the exhaust turbine

I find that interesting. Do you have a link to data?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[RVAmeche]

It appears I misremebered the benefits. According to the Mercedes engineer (youtube link below, starts ~0:45) the real benefit is including the electric power unit between the split compressor/exhaust turbine, which allows for better packaging and response. The Jalopnik article and comments has some other interesting info.

Jason @ Engineering Explained also did a pretty good video on it discussing various aspects.

https://www.youtube.com/watch?v=l2_eTLCz_3I

https://jalopnik.com/how-mercedes-ingenious-f1s-split-turbo-works-1560969552

https://www.youtube.com/watch?v=ckCd9D-eBm4

I think I was remembering a post similar to this one by Turbo Dynamics below that claims the main benefit is the separation from heat. Based off the other sources, it appears the more efficient/aerodynamic packaging options of the split turbo likely supersede the temperature benefit.

https://www.turbodynamics.co.uk/media/blog/mercedes-split-turbo/

 

[gruntguru]

To my eye the main benefit is packaging of the compressor. The "pancake" space available in the MB layout allows an efficient compressor/volute design with little constraint on size.

je suis charlie

 

[hydrae]

One item not mentioned yet
This is a farm tractor, where draw bar pull is the result of the horsepower and is the critical performance characteristic.
The smaller turbo engine will be lighter so to get the same pull as the larger engine dead weight would have to be added.
Also turbo lag is not an issue in this application, again it is a tractor which runs close to full throttle most of the time.

Hydrae

 

[PackardV8]

One item not mentioned yet
This is a farm tractor, where draw bar pull is the result of the horsepower and is the critical performance characteristic.
The smaller turbo engine will be lighter so to get the same pull as the larger engine dead weight would have to be added.
Also turbo lag is not an issue in this application, again it is a tractor which runs close to full throttle most of the time.

Hydrae

On the mass of a farm tractor, the weight differential between the two engines is antshit. Agree, turbo lag isn't a factor. Agree, but would suggest farm tractors pull at torque peak, as defined by EGT. On a diesel the difference between horsepower peak and torque peak is smaller than on a typical gas burner.

jack vines

 

[Compositepro]

Tractors usually have ballast weights because the amount of pull force it is capable of depends as much on the weight of the tractor as it does the power of its engine.