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The Myth of Compression Braking


Recently a remark by an acquaintance triggered me to dredge up a bit of knowledge I came by ages ago, during a short stint driving a diesel tractor/trailer rig for J.C. Penney between Phoenix and Tucson. The fellow, a UK resident, had recently sold his mid-90s Land Rover Discovery V8 with a five-speed manual transmission, and purchased a similar Discovery equipped with a 200 Tdi turbodiesel engine, also with a five-speed. Besides instantly doubling his fuel economy and then some, he’d noticed something else and mentioned it in an email. “I might be daft,” he wrote, “but I swear the petrol engine had better compression braking. But I know that’s impossible. It only had 9:1 compression, and the Tdi is 19:1. Something else must be at work.” 

I wrote him back a short reply: “That’s because diesel engines have no compression braking.” Which caused him to respond, “Now I think you’re daft.” 

My next email was much longer, and read something like this: 

First, the term “compression” braking is a misnomer. While there are several forces at work (including simple internal friction) when a driver lifts off the throttle in a vehicle and the engine slows it, the force often referred to as compression braking is more accurately called vacuum braking. It occurs in a gasoline engine because the throttle, i.e. the gas pedal, works by regulating the amount of air entering the engine—the fuel-to-air ratio is kept relatively constant. The air flow is controlled by a rotating plate or series of plates in the fuel injection’s intake system (or, in older vehicles, in the carburetor). When you lift off the gas pedal, that plate closes off the intake nearly completely. The engine, which is still turning at speed via its connection to the turning wheels, then has to suck air past the closed plate, and that retards the engine, and that is what allows our manual-transmission, gasoline-engined vehicles to creep down quite steep inclines in first gear low range, without the need for brakes. 

A diesel engine is different. In a diesel, the throttle controls the amount of fuel being injected into the engine, rather than the air flow. The air intake system in a diesel is always fully open. Thus, when you lift off the pedal in a diesel-engined vehicle, the fuel supply is reduced, but there is no vacuum effect to slow the engine. Thus, no “compression” braking. 

You might ask, but what about the air still being compressed in each cylinder as the piston rises on that 19:1 compression stroke? Doesn’t that retard the engine? The answer is, yes, it does; however, once the piston passes top dead center, that compressed air is still pushing against the piston, only now it’s trying to speed up the engine, even without enough fuel injected to produce real power via combustion. So the two forces essentially cancel each other (the same effect applies to gasoline engines). 

 Opening photo: A 3.8-liter gasoline engine. This photo: A 3.0-liter Turbodiesel. Which one has more engine braking? 

 This lack of what we really should just refer to as engine braking is why many big diesel trucks, such as that J.C. Penney tractor/trailer rig, employ what is known generically as a Jake brake, after Jacobs, the company that originally manufactured the device. A Jake brake opens the exhaust valve on each cylinder at the top of the compression stroke, so the compressed air is released out the exhaust (with a machine-gun-like hammering that prompts communities to post those signs prohibiting their use within urban areas). That leaves just the compression stroke working to retard the engine—so a diesel truck with a Jake brake really does have compression braking. 

The exhaust brake is another, quieter (although generally less effective) method of engine braking on a diesel truck. This device partially closes off the exhaust, so the air pushed out through the exhaust valve on the exhaust stroke is restricted, slowing the engine and the vehicle. Some trucks now employ both Jake and exhaust brakes to maximize efficiency while reducing noise. 

So that’s why an engine with more compression can have less “compression” braking. 

I checked: Unfortunately, Jacobs doesn’t make a model for the 200 Tdi . . .


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Reader Comments (9)

Great explanation! I just learned something new. Thanks!

October 5, 2011 | Unregistered CommenterTim

Great explanation of engine braking. But what it does not answer is why the owner of the diesel Discovery feels the vehicle has more engine braking in spite of the fact that diesels don't actually have it. Is it simply a matter of gear reduction while in low range? After reading this explanation one might surmise that an overland truck with a diesel engine would rev uncontrollably on steep descents while in low range, but this does not happen. Do large freight trucks need Jake and exhaust brakes simply because they don't have low enough gears? Of course even if they did have these gears it would not make economic sense for drivers to use them given the extremely low speeds it would create. Time is money as they say.

October 5, 2011 | Unregistered CommenterTroy M.


Read it again. He said "I swear the petrol engine had better compression braking".

October 5, 2011 | Unregistered CommenterLarry H.

Sorry my mistake. But the question is still valid, given diesels don't have engine braking why do they not rev excessively on steep descents while in low range. Again, this question is specific to small overland trucks. Freight trucks obviously do rev excessively on descents without other braking mechanisms. Is it a function of the gearing or simply a matter of the extreme weight of freight trucks verses the light weight of overland trucks by comparison?

October 5, 2011 | Unregistered CommenterTroy M.

Thanks for the comments, gentlemen. At the moment I'm in Tanzania, having just experienced the myth of compression braking on a 110 300Tdi while descending the wall of the Rift Valley with a distinctly soft brake pedal. So more later, but: as I understand it, simple internal friction in the drivetrain accounts for much of the retardation one still experiences going down hill in a lightweight vehicle equipped with a small diesel engine.

October 16, 2011 | Unregistered CommenterJonathan Hanson

I think Jonathan is right about the friction reducing the speed of a diesel without the addition of a Jake or exhaust brake. All three of our vehicles that are currently on the road are diesels - a 1965 Land Rover IIa, a 2000 VW Golf TDI, a 2005 Jeep Liberty CRD - plus I drive a Freightliner Sprinter daily for work. Each vehicle does have added resistance when shifting to a lower gear on a downhill, however minor compared to a petrol version of the same vehicle. In fact, in my CDL training it is always advised that on a downhill you select "the same gear that you would use coming up the same grade." (E.g. If you have to use third gear to get up the hill you select third gear coming down the hill.)

There must be some huge physics explanation involving somebody's law, but, as it has been explained to me over the years, the fact that you have gears and shafts and the like spinning through fluids in the transmission will create drag, or friction. Since warm liquid flows better than cold liquid I guess there would more resistance at the beginning of the downhill than at the end. Possible one reason transmission coolers are used on big rigs and towing vehicles.

So, if the engine was connected directly to the wheels without any type of transmission in between, from what I understand, a diesel would spin without restraint on a downhill.

Now, how about a two stroke engine?

October 16, 2011 | Unregistered CommenterTad

Oh, Troy, you are correct that since a semi truck fully loaded weighs like 40 tons compared to a fully loaded overland rig weighing 2 to 3 tons, weight is a distinct DIS-advantage to the big rigs. That is one reason truckers will use all the tools in the box when descending a mountain pass.

Keep that in mind next time you decide that there is enough room to pull out in front of that tractor trailer!

October 16, 2011 | Unregistered CommenterTad

"You totally neglect the heat generated in the compression cycle. Any heat increase is a loss in pressure (P*V = nRT). When we decrease volume (compress), both the Pressure and Temperature increase.

My example is for a 2L diesel motor, with say a 19:1 compression ratio. That should get you somewhat over 600 degree increase in cylinder temperature.

Temperature increase 600 Degrees
Volume/cylinder 500 mL
Specific density of air (kg/m3) 1.184
Volume 0.0005 m3
Mass of air 0.000592 kg
0.592 g
Specific heat capacity of air ( kJ/(kg.K) ) 1.047
Energy consumed (per revolution) 0.3718944 kJ
Revolutions 2000 /min
. 120000 /hr
Total energy 44627.328 kJ/hr
Convert kJ/hr to kW divide by 3600
. Power 12.39648 kW
So in this example we would have the equivalent of a 12 kW motor acting as a brake."

boost pressure from turbo will enhance braking, as will DPF.

There is engine braking with modern diesels, not as much as petrols and not due to the same type of mechanisms.

February 4, 2014 | Unregistered Commenterreposted from whirlpool

Very interesting information, thank you. However, I didn't ignore the heat generated in the compression cycle; I simply assumed that, unless my grasp of physics is off, it's nearly inconsequential. The heat is produced by the compression of the fuel/air mixture, and of course in a diesel engine that mixture is compressed and heated enough to initiate ignition without the need for a spark plug. Nevertheless, it seems apparent that the vast majority of the pressure generated in the diesel compression stroke, thus inhibiting the rise of the piston (so-called "compression braking") also pushes against the piston once it passes top dead center, thus counteracting any "braking" effect. If the pressure (and heat) had a significant effect, would not engine braking in a diesel - especially a turbodiesel - be demonstrably greater than that in a lower-compression, lower-temperature gasoline engine? Yet we agree this is not so. So it seems to me, accepting your excellent calculations, that if the mechanism you propose produces greater pressure against the piston on its upward travel than it does on its downward travel (which is how I read your post), the difference is negligible despite the 12kw figure. Therefore I think it's still clear that it is the difference in throttle systems between gasoline (air-controlled) and diesel (fuel-controlled) engines that is the primary reason gasoline engines display greater engine braking. Counterpoints welcome!

February 4, 2014 | Unregistered CommenterJonathan Hanson

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