Engine Braking

[Dexterpix]

When you shift to a lower gear the braking effect is from the engines mechanical resistance, not from inside the transmision. The transmision does not create enough resistance to have a braking effect. A transmision disconected from the engine can easily be turned over by using a wrench by hand. The engine without heads in place can also be easily turned over by hand/wrench. When the heads are in place and compression is created then you have to overcome the resistance of the engines compression to rotate the internal mass of the engine. The high the compession the greater the resistance. This creates the braking effect. All interernal engines need a device (plate, buttefly, throttle body) to regulate the incoming air that is the basic function of an engine (air in = air out) restrict either one and you choke the engine from functioning. Jake brakes are a company name that has become the genral name of any exhaust brake added on to the exhaust side of the engine. Most modern large diesel engines employ "Compression braking" and utilize variable valve timing to restrict the exhaust flow out of the heads prior to it entering the exhaust manifold. Compression resistance is the only mechanical resistance strong enough when combined with gearing to give the braking effect. Yes if you do not shift to a lower gear the rolling resistance will not increase the engine rpm enough to effectively cause the braking effect if you do not have an exhaust type brake installed. Down shifting will increase the RPM and thus the resistance of work load the engine can product.

p.s. I have worked on heavy equipment diesel engines for over 30yrs.

[Steve & Mary]

I recently had to use the tow/haul feature on my F-250 diesel when my Dexter 1600 actuator failed and I had to disconnect power to it which left me no trailer brakes. We were in the Ozarks in Arkansas and I had never had a need to use the tow/haul feature. I was really surprised at how well it worked. It worked so well the RV behind me (my 85 year old dad and mom)complained my brake lights were not coming on enough and it was really messing him up on when to brake. These were the first hills we had really gotten into with the F-250 and our 31ft. Classic. I felt totally comfortable in the ability of the truck to handle the trailer without any trailer brakes. I made sure I allowed for extra distance when in traffic. Since then I have replaced the brake actuator and all is good :-)

[jcl]

Originally Posted by Dexterpix

[COLOR=black] All internal combustion engines have some type of "throttle plate" you have to regulate the intake of air into the engine. More air = more fuel = more power. A diesel without a "exhaust brake" will still use the engines compression to resist acceleration. Diesel engines are a more effective engine brake because of the higher compression (2x or more than most gas engines)

Originally Posted by jcl

All spark ignited engines have some form of throttle plate. Not true for compression ignition engines. Thus, diesels have no significant retardation effect unless fitted with an auxiliary brake such as a Jake or other design. Compression ratio doesn't enter into it as the work to compress air on the up stroke is returned on the piston down stroke.

Originally Posted by Dexterpix

When you shift to a lower gear the braking effect is from the engines mechanical resistance, not from inside the transmision. The transmision does not create enough resistance to have a braking effect. A transmision disconected from the engine can easily be turned over by using a wrench by hand. The engine without heads in place can also be easily turned over by hand/wrench. When the heads are in place and compression is created then you have to overcome the resistance of the engines compression to rotate the internal mass of the engine. The high the compession the greater the resistance. This creates the braking effect. All interernal engines need a device (plate, buttefly, throttle body) to regulate the incoming air that is the basic function of an engine (air in = air out) restrict either one and you choke the engine from functioning. Jake brakes are a company name that has become the genral name of any exhaust brake added on to the exhaust side of the engine. Most modern large diesel engines employ "Compression braking" and utilize variable valve timing to restrict the exhaust flow out of the heads prior to it entering the exhaust manifold. Compression resistance is the only mechanical resistance strong enough when combined with gearing to give the braking effect. Yes if you do not shift to a lower gear the rolling resistance will not increase the engine rpm enough to effectively cause the braking effect if you do not have an exhaust type brake installed. Down shifting will increase the RPM and thus the resistance of work load the engine can product.

p.s. I have worked on heavy equipment diesel engines for over 30yrs.

Well, the OP wasn't asking about a diesel, but this should be corrected.

Agree the retardation effect is from the engine, not the transmission.

However, compression doesn't enter into it (unless a compression release brake is employed) since the work to compress the air in the cylinder is returned on the downstroke. It is just a spring.

With a gasoline engine, the air restriction at the throttle plate provides a choke that creates retardation. It takes work to force air past that restriction.

Diesels simply don't have that restriction. As much air as ambient air pressure, or the turbos, can force into the cylinder, enters the cylinders. Fuel is controlled for purposes of throttle, not air volume.

Compression only matters for retardation if there is a compression release around TDC, like a Jake brake, to release the compressed air. Not applicable to trucks towing Airstreams.

Modern diesel pickups wil have next to no retardation from the engine itself. They may have an exhaust restrictor, factory or aftermarket, that works like a choke to create retardation, but on the exhaust instead of the inlet.

Modern diesel pickups with pollution controls such as EGR and particulate traps have inherent restrictions due to these devices, making them act more like a gasoline engine. There is also the effect of any variable vane turbos. But this is exhaust braking, not engine braking, IMO. Perhaps not a significant difference.

PS. 24 years in the Car dealer network, plus time spent developing fuel systems for over the road (class 8) and larger off road trucks.

[idroba]

Originally Posted by BoldAdventure

Actually, the 5.7L Hemi Ram features a form of engine braking, basically the computer tells the transmission to down shift automatically when in tow/haul mode when the brake is applied. It works incredibly well.

Since most folks ignore the USE LOW GEAR signs, it basically does it for you, thus slowing the vehicle. But only when tow/haul is engaged. It will do it without apply the brake too, I'm not sure how it senses it. But coming over a hill today, I took my foot off the gas and the engine brake engaged while going down the hill.

My 2102 Jeep Grand Cherokee with the 5.7 hemi and 6 speed auto has the tow/haul mode which I use virtually all of my towing time. I live in an area of long steep grades. It is amazing to me how well the braking works, all automatically. I come over a pass crest and down the other side. I touch the brakes lets say at 45 mph, and the Jeep seems to automatically try to keep that speed as max by automatically downshifting to hold the speed. Occasionally I have to further brake, but for the most part the tow/haul setting and how the computers are set do all the slowing by gear shift and compression.

I went down a 6/7% 10 mile grade with the FC 20' behind me and only touched the brakes twice more after the initial time. I have no idea what gears were being used, as that does not show up on any readout, but the effect is very impressive.

If you have a Tow/haul mode on your rig, no matter who makes it, give it a try to see if it essentially does the work for you. The Jeep certainly does.

[ShinyPete]

Idroba is right on. Towed our Pete a 27fb 150 miles today with our 2014 F150 5.0 over some Florida grades 😄. Just put it in tow/haul tap the brake going down and let it handle the shifting.


Sent from my iPhone using Airstream Forums

[AlbertF]

Originally Posted by sandrade

So, my questions are....how does engine braking work? Can ones engine really survive the RPM's of being pushed down hill by a TT? What is the best approach to truck brake, engine brake, and trailer brake combo?
Came down the Big Horn Pass, WY this summer and was shocked at how much gravity influences the whole rig. I mostly kept it in control. At the bottom ran into another RV'er who proclaimed his awe and said he was turning around and going back to Maryland after that ordeal. I rode my rig down in 2nd gear, avoiding 1st in fear I would explode the engine. Alternated truck and trailer brakes, but avoided all out foot stomping. I have a 2013 Ford F150 8 cyl. auto, 2013 International Signature 23'. Any recommendations about how to safely handle steep downhill grades without burning out the brakes? thanks!

When you live in flatland areas, you can avoid ever revving a vehicle's engine past 2500 rpm, if you drive gently. It's very different in the mountains. We have gotten quite accustomed to holding lower gears with the engine spinning in the 4000+ rpm range while climbing or descending, and no longer think anything of it. I have never detected any harm as a result.

You probably needed first gear on that descent. I think you would find a major difference between 2nd and 1st in terms of engine braking. Crest a hill slowly, so you have time to manage your speed and determine what gear you need to be in. Mountain driving is not difficult or dangerous, as long as you are patient and thinking ahead. And don't worry about being slow; there are lots of trucks and older motor homes that are slower than you, and the locals will understand.

[ROBERTSUNRUS]

Hi, my Lincoln does a good job of engine braking; I have gone down many mountain roads with little or no use of my brakes to maintain a safe speed. In the past, I have done things with automatic transmissions that would make you cringe. You can manually shift your trans into first gear at 100 miles per hour and it won't hurt anything. It will down shift to a proper gear at the right time and not sooner. You can also lock your trans in first gear while going down a steep hill and when your engine hits a certain RPM it will upshift on you.

My new F-150 Eco-Boost has Tow / Haul mode and will down shift when you touch the brakes going down hill to help maintain your speed. I don't have a new shank yet, and I don't have enough miles on it for towing duty yet. It has been a concern as to how much engine braking the 3.5 L engine will do. My Lincoln's 5.4 L engine holds my trailer back just fine. I will report on how the Eco-Boost does when going down hill when I get to that point.

[Andrew T]

The last time I towed an Airstream with a V8 in the mountains was about 30 years ago everything since has been with a V6 think you will find you need the run the 3.5 about 6-800 RPM higher to get the same amount of braking from it.

The two extra gears and the lower first gear ratio will give you a better operating range and first gear will give you just as much total breaking.

[ROBERTSUNRUS]

Originally Posted by Andrew T

The last time I towed an Airstream with a V8 in the mountains was about 30 years ago everything since has been with a V6 think you will find you need the run the 3.5 about 6-800 RPM higher to get the same amount of braking from it.

The two extra gears and the lower first gear ratio will give you a better operating range and first gear will give you just as much total breaking.

Hi, Thank You; That makes sense, smaller engine = higher RPM.

[SCOTTinNJ]

V6 likely has higher rpm range anyway.

[SCOTTinNJ]

deleted.

[DKB_SATX]

Originally Posted by Dexterpix

All internal combustion engines have some type of "throttle plate" you have to regulate the intake of air into the engine. More air = more fuel = more power. A diesel without a "exhaust brake" will still use the engines compression to resist acceleration. Diesel engines are a more effective engine brake because of the higher compression (2x or more than most gas engines)

Always followed a simple rule that whatever gear I needed to climb a hill was the gear I used to descend the other side. Both climbing and descending are fighting gravity, in theory the work load to maintain a given speed should be similar.

Originally Posted by jcl

All spark ignited engines have some form of throttle plate. Not true for compression ignition engines. Thus, diesels have no significant retardation effect unless fitted with an auxiliary brake such as a Jake or other design. Compression ratio doesn't enter into it as the work to compress air on the up stroke is returned on the piston down stroke.

The blanket statement that all spark-ignition engines have a throttle plate is no longer accurate, and is likely to become less generally accurate as technology moves forward. BMW's "Valvetronic" intake system has a sort of a choke plate in the intake stream but it remains fully open once the engine is running at normal temperatures and the engine manages air intake by varying the intake valve opening. I'm not sure if Fiat's "MultiAir" system even has the choke plate in the stream, but it runs in a similar fashion to the Valvetronic concept.

The even more general "all internal combustion..." statement is way off, since (as stated several times up-thread) there's no need for a throttle plate in a diesel.

Also pay heed to what "Al and Missy" had to say: do not fear downshifting, a modern automatic isn't going to shift into a gear that'll ruin the engine, it'll wait until you're going slow enough. Shift down for whatever braking the drivetrain can offer you at that moment and keep scrubbing off speed as you descend. It may be noisier than you expect (just like climbing the hill you just crested probably was if you have a NA gasoline engine like my 5.4L) but it's part of what your vehicle was designed to do.

[jcl]

Originally Posted by DKB_SATX

The blanket statement that all spark-ignition engines have a throttle plate is no longer accurate, and is likely to become less generally accurate as technology moves forward. BMW's "Valvetronic" intake system has a sort of a choke plate in the intake stream but it remains fully open once the engine is running at normal temperatures and the engine manages air intake by varying the intake valve opening. I'm not sure if Fiat's "MultiAir" system even has the choke plate in the stream, but it runs in a similar fashion to the Valvetronic concept.

Appreciate the correction. I actually have a BMW with Valvetronic, and have towed small trailers with it, but thought we were getting a little deep into motorhead territory.

Add Toyota's Valvematic system to the list.

Jeff

[perryg114]

Well if they electronically control the valves there is no reason they can't build in some sort of engine brake into the software. I try to stay at least 10 yrs old to avoid lemons in the R and D process, let fools rush in.

Perry

[jcl]

Originally Posted by perryg114

Well if they electronically control the valves there is no reason they can't build in some sort of engine brake into the software. I try to stay at least 10 yrs old to avoid lemons in the R and D process, let fools rush in.

Perry

If you are referring to Valvetronic, you can safely jump into the pool with us early adopters then. It has been in production since 2001.

I do understand your sentiment. With technology-heavy vehicles like BMWs I avoid the first production year of new models.

[Hodum]

I am certainly glad that i read this thread from front to back. However most of it was far too technical for me, but I have a F 250 with an exhaust brake that worked like a charm on the Blue Ridge parkway. It is up one and down one then all over again. I don't understand the process, but with a thirty four foot Classic loaded to 10,300 lbs. it told me all I needed to know.

[tjdonahoe]

Too technical for me too ,and I have a dodge 2500 with a 6.7 with an exhaust brake and 6 speed automatic and it has towed our as's 50000 with no problems and I also don't worry what the weights are, we just go....no worries..big is sometimes better.....

[Aerowood]

I don't even think about it anymore, downshifting in the Colorado mountains is just what you do, trailer or no trailer. When going up the passes I don't pay any attention to the speedometer and drive the tachometer. I couldn't tell you how many times trucks and trailers and even cars wipe themselves out by not downshifting going down the mountain passes. I think some people rely to much on technology instead of paying attention to the environment them. Safety is not just throwing a switch on the dash.

[tjdonahoe]

You are correct, you slow down on the down side,I have been to Ouray up and down the million dollar highway and over to Silverton and Durango.It sure does feel good with the exhaust brake on, you seldom have to use the service brakes..my big truck gets 550000 miles between brake replacement.you should try a tv with an exhaust brake, you might like it....

[wvstreamer]

I have a similar tow vehicle. A 96 7.3 F 250 SC. I have not investigated too far but my son had one that used the factory butterfly in the exhaust that is the warm up method from the factory. The device was some sort of programmer/software that was plug and play. It closed off the exhaust and locked the converter. Worked well and cost was reasonable. If I learn more I will let you know. The old 7.3 is hard to beat-towing or not. I am at 160K and with good care, plan on another 160K.