Where Does Engine Braking Come From?

Have you ever tried turning an engine over by hand?

You're on the right track. Gas engines have throttle valves which control the air flow and coincidentally the manifold pressure. The engine has to compress the air from this low pressure to something above atmospheric to get it out the exhaust. Diesels don't have throttle valves, so the engine braking is less. However, both have frictional losses, bearings, pistons, rings, gears, cams, air flow through valves and passages, oil churning, blow-by, etc.

Gas engines use compression, and since there's no gas to burn, the engine becomes an air compressor.

Diesels use a "Jake brake".

Compression release engine brake - Wikipedia, the free encyclopedia

Some guy named Jake.

The Jake brake is named after I assume the inventor and it is called the Jacobs Engine Brake. Mack had their own system but I cannot remember the name they applied. Dyna Brake I think.. Jacobs makes kits for most engines. Some manufacturers even make kits for diesel pickups that consists of a valve on the engine exhaust system down stream of the turbo. Quite effective depending on the brand from reports.

It is caused mainly by:

1. Inequality of compression and power strokes: a lot of compression work is not returned during expansion. (Think about why a compressor becomes hot.)

2. Friction: the higher the ring tension, the more engine braking from friction. (And of course all other sources of internal engine friction play a role. Also external sources, --- even transmission friction, tire rolling resistance, etc have the same "driver apparent" effect as "engine" braking. Lift off the throttle and the vehicle slows due to all the frictions involved.)

3. Pumping losses: moving air past a mainly closed throttle requires work. (This does not apply to diesels, in most cases -- the "solution," if engine braking is desired, is the jake brake, which wastes the compression energy almost entirely, by turning the engine into an air compressor.)

Engine Breaking from "Spanner in works"

Regards,
Sapper

Engine braking is to be avoided unless the hydraulic brakes and emergency brakes have failed.

In effect you starve the engine of fuel, so instead of producing power, it compresses air and then vents it and compresses more. With a stick shift you can maximize engine braking. Bear in mind, you can also make a broken engine or clutch this way, say by dropping it into second gear at 90 miles per hour, which might well take the engine far past the red line...

Engine braking is to be avoided unless the hydraulic brakes and emergency brakes have failed.

NO! NO! NO! Use of engine braking SAVES the service brakes by keeping them cooler, thus better able to perform in an emergency. Some government entities do not realize this and put up signs "No engine brake" when they really should enforce their noise ordnances. It is mainly the truckers who have "messed" with their mufflers that produce most of the noise. Properly muffled, the Jacobs Brake and other engine brakes are not too noisy especially when you consider that they are contributing to safety.

By the way, there are other types of brakes based on eddy currents, oil pressure, and maybe others that I haven't heard of. The Thelma Retarder is one of these.

There seems to be some confusion, slowing down by removing your foot from the accelerator provides a slight braking effect, typically 1/20 g or so. A hard stop is about 0.9 g and can exceed one g at times.

Changing to a lower gear so the engine is forced to high revs (but not red line) also provides braking.

The gradual braking, is fine. trying to use the engine for hard braking is not wise.

In an emergency, braking of all kinds is acceptable.

Your last statement is not true. Using low gear induced engine braking in an emergency introduces too many variables and can make the situation worse. In both car, rigid HGV and articulated HGV training, I was taught to hit brake then clutch pedals (in about the time it takes to say that) when driving manual shift. That way you get some initial engine braking contribution, but then remove all drive to the wheels. This is especially important in an articulated rig where the risk of wheel lock and jack-knifing exist. You can then concentrate on staying straight and feeling for lock-up (I learnt in vehicles without ABS) and be ready to cadence brake.

An Advanced Driver once told me that a similar theory exists when driving in snow. I have to admit I tend to drive on engine braking and use of momentum in snow/ice. However, if the car begins to slide, I take that AD's advice and depress the clutch, thus removing motive force and allowing me to concentrate on steering and gentle braking.

At the other extreme, when my master cyclinder failed and I had to drive the car to the garage, engine braking and (ineffective) hand brake force was all I had! Combined with lots of anticipation and looking ahead, I got there safely (if a little damply!)

In an emergency, braking of all kinds is acceptable.

In an emergency, a car driver should do nothing but apply the brakes with full force. Fiddling with the clutch and gearshift just wastes time. This is why Volvos and other cars have brake assist to help a panicked driver apply the brakes harder than they normally would.

What you wrote is mostly wrong or misleading or downright dangerous. Do you actually have a driving license?.

Everyone should learn how to use the engine as a brake just in case you get a brake failure on a downhill. You don't want to start learning when they fail.

Also engine braking means that on long down hill drives, the brakes will not need to be used/overheated and they can remain cold and unused for any possible emergencies or for final stopping.

If in an emergency the motor gets over revved, who cares? Better to have an engine repair rather than a car wreck! Though I personally have never heard of an over revved engine in such cases, where next to no fuel is being burnt, actually getting damaged.....but I expect it to be occasionally possible......

Over revving with the throttle/power is far more dangerous to most engines.....though most modern ones have a rev limiter built in....though it would not help on a hill.

Driving cars for many years towing either heavy trailers (with a car on them) or caravans has demonstrated the need for correct usage of engine braking to save the brakes from overheating. Its simply good driving practice.

Learning how to cause an automatic gearbox to select and retain a lower gear on a hill is also a good idea in such cases that many have never learnt......how dangerous of them!

In Europe, there are often road side notices informing truck drivers (and fully aware car drivers) that a steep hill is coming up and to make proper preparations for it.....The US too if I remember correctly.....

I hope you now fully understand now where you went wrong in your post !!!!!!

Also I hope you take the time, for the safety of yourself and your family, to learn how to use the engine to brake on steep hills instead of using the brakes.....it is important.

The situation you mention is simply bad driving, of which many are guilty.....

there's videos on you tube that explains how an engine brake funtions.

Maybe in an automatic, but you'd have to have a death wish to go down a 25% incline in a high gear with a manual box.

Disagree - far from relics of the past. On a serious drive with a lot of inclines etc engine braking is the way. Saves a fortune in brakes pads and servicing. You are right in what you say about diesels - they cannot engine brake unless modified. Can't help thinking if it was such a bad idea the big diesel engine manufacturers wouldn't have bothered engineering them and installing them into trucks and buses.

Brake pads are far cheapeer than engine repairs

No. You are wrong. Take a look on a engine manufacturer site to see. braking is standard and usually independend of driver desire.

agree I would rather replace a set of brakes than an engine but the engine is turning already - might as well keep it busy. Helping the engine tickover has to reduce the gas you use - you still gas to keep it ticking. I'm not suggesting you drop the car into 2nd gear at 70mph to slow things down so engine damage is extremely unlikely.

What?

Would you please cite some source supporting this opinion?

Lots of references on google about risks of damage to the power train and wasted gas.

Many years ago, when drums ruled and faded, there was value in EB. Now with modern disk brakes it is not the best choice. It does remain as a last resort in case of brake failure.

You are right about brakes coming a log way in cars and light trucks.

And nobody's going to downshift their automatic transmission to slow down. Probably not their manual transmission, either. But even if they do, I say no damage will be done to the engine/drivetrain.

You are dead wrong about heavy diesel rigs.

In a manual, you have to change down before going into a bend (if you're doing any reasonable speed and expect to come out alive), so it's inevitable that you use engine braking - in combination with disks or whatever as necessary.

Does noboby include me? I teach my daughter (new driver) the same thing. It's in the owner's manual for both of my newer cars as well.

I never like that argument about replacing brake shoes is cheaper than engine repair...I don't jam it into a lower gear and hit the rev limiter for Pete's sake.

Of course, I check my tach before downshifting...in an automatic...guess I am weird that way.

Ok, Almost nobody.

I have some experience with vehicles with and without retarders in fire and rescue service for the last 25+ years. The local county was providing emergency medical service in Dodge 1 ton cutaway vans. There were so many accidents from running into the back of other cars that the county installed push plates on the next series of trucks (Dodge 3500 chassis cabs with a big box on the back) to cut down on the damages to the rescue truck. The next series of rescue trucks were Econoline vans with Thelma electronic retarders installed in back of the trans. We learned a few things from this experiment.

1) we had 2 extra trucks on any given day. That is the number of trucks that were getting brake jobs EVERY day.

2)We learned that we needed a maintenance program for wheel bearings now. Previously, brakes were good for only 12-15 hundred miles. And rotors, calipers wheel cylinders and drums were every other brake job.

3) The rear axle bearings would wear out faster with the Thelma retarders. They would wear out in 8-12 months. Switching to synthetic gear oil and changing it every other engine oil change helped out a lot with that.

The retarders installed as part of the heavy duty Allison transmissions are a lot smarter than any one gave them credit for. My first transmission retarders were installed on trucks with 66,000 lbs GVW The retarders were applied with a separate pedal. The Allison service rep had to work with us as the initial transmission filters would blow the filters off of the filter housings. After a filter design change we were instructed to use the retarder by pulling the shifter into first gear, and standing on the retarder pedal. These trucks topped out at 68+ mph. When shifted into first gear at any speed, the transmission would inhibit the downshift until the engine speed in the next lower gear was below governed speed. The truck would drop each gear in turn as the engine speed would allow it. This was an "output" retarder that required downshifting. Other designs are "input" retarders that retard the input shaft, and do not require downshifting. These transmissions were pre electronic. Straight hydraulic shifting.

Todays Allisons communicate with the ABS system, and will not cause lockup in the rain. Snow, is not a condition that I can speak of regarding ABS and transmission retarders. But it should also be the same.

Well, I may have jumped in too early. I need some coffee!

I have read again the thread. I first read the comments in the muddled mindset of compression release braking, not engine braking. So there is merit to your comments; your reference to engine braking being ineffective in diesel engines discounts the low RPM available in them, thus low gear selection on steep downhill travel will be effective.

However, I should not have challenged you on your comment.

Please chalk this up to my foggy thought process.

You still didn't cite actual passages! Please do so.

In my opinion, and others here, you are wrong.

In the manner he means, he cannot cite any useful links, or he would have done already.

I would expect there are some where people have panicked and smashed a gearbox into a far too low a gear.......not exactly a good thing to do as anyone here could probably agree to....

I looked around on the web and found this at:-

http://en.wikipedia.org/wiki/Engine_braking

With diesel engines however, there is no intrinsic engine braking effect so more care must be taken. Turbo-diesel engines, on the other hand, generally have a more noticeable engine braking effect due to the turbo stalling when the accelerator is released and increasing the back-pressure in the exhaust.

....in a Wiki article. So some diesels are "braked"!!!!! I knew it happened, but could not explain it for CR4, as I have driven many turbo diesels over the last 30 years, always selecting a lower gear and maybe just occasion dabs at the brakes for corners and the like.

But I have only driven a few non Turbos (many years ago in the RN), but always with retarders on long hills.....

I did find this dangerous comment which will mislead some:-

http://answers.yahoo.com/question/index?qid=20060624014115AA4Huop

As a motor mechanic and driving for 57 jears....I am sorry to have to tell you that you do not know much about engines or driving.

And...diesel engines do provide "engine braking" even without the exhaust brake although less than gasoline "petrol" engines.

Nothing wil happen to the engine or clutch as long as you keep the revs within the engines limmets.

Even with an automatic box, one should gear down if you carry a heavy load and/or going down a steep hill.

For crying out load.....how does gearing down waste feul!!!!!

Please stop talking nonsense! You do not seem to understand what many are trying to tell you.

and as said, useless on diesels.
Engine braking is highly effective on diesels equipped to do so.
Do you have some reference that shows that all the truck drivers and engine brake manufacturers are wrong?
Perhaps you are thinking of roadracing where downshifting a petrol engine is now rarely used for slowing the car, but was until the 70's?

There are a few people responding that do not realize there are two types of engine braking. Professional truckers know about Jake brakes. Others may not. Normal engine braking without Jake brakes depends on restricting flow of fuel which reduces speed by engine not "putting energy back onto the shaft". This type of "braking" is similar to coasting to a stop compared to Jake brakes.

I'll throw a real spanner in here. Although I am exponent of engine braking I also know to my cost that in slippy conditions and I'm thinking here of snow and ice it isn't the way forward in a small car. In a low gear taking your foot off the acelerator will cause the car to slow down through engine braking. A car seldom drives through both wheels and will brake from one wheel. As your driven wheel turns into your braking wheel your world will gently begin to spin. Most effective at 10 to 15 mph. In these conditions do not engine brake but always dab the foot brake gently.

I try not to use brakes to slow down, only for stopping.

Thanks to all who replied- it's starting to veer away from what I originally wanted to establish, but the answers, as I understand them are: On a petrol engine, or a diesel with a butterfly valve, engine braking comes from restricting the gas flow. Engine braking can also happen when the compression is "wasted", either intentionally in Jake brakes, or as a result of poor engine compression. The bit that I was struggling to get my head around was that as the compression gets worse, you get, AIUI *more* engine braking. Where to go with this? Since it's now a fairly simple matter to measure and record vehicle acceleration and deceleration with a smartphone, it should open up new quick and easy checks of vehicle condition.

Well,

Close enough.

But, measuring power loss due to loss of compression as a result of performing acceleration tests would be meaningless.

Since you are not in a test cell, there is no way to control the test conditions. Wind, grade, fuel, tons of other variables that are controlled when looking for an accurate evaluation of loss of power, due to loss of engine compression, due to the loss of the factory sealing abilities of pristine rings/cyliders.

It's too slow a process to measure this way, anyway.

On a petrol engine, or a diesel with a butterfly valve, engine braking comes from restricting the gas flow.

Your understanding may be enhanced by considering just one engine configuration at a time.

There are many effects that add together to create what is perceived as engine braking. Consider a modern petrol engine car with a manual transmission, cruising on a flat road.

When you lift off the throttle, (typically) fuel flow is shut off entirely. If the car was cruising at a speed that required 20 hp and the engine is now producing -5 hp, the car must slow. Some of the slowing is from the engine, and much of it is from other sources. (If you disengage the clutch, the car will continue to slow at a lower deceleration rate.)

The -5 hp comes from engine drag: If the engine is not old, ring tension is high, so the engine is hard to turn for that reason. Seal tensions are also high, adding to the work required to turn the engine (using up the car's momentum). Each compression stroke generates heat, which is rejected to the radiator as waste heat. Each intake stroke requires the piston to move against a pressure differential, and the air entering the cylinder does so turbulently, causing heating -- energy extracted from the car's momentum. Each exhaust stroke requires the pumping of air. The air conditioner and alternator both extract varying amounts of energy from the car's momentum.

Then there are all the things that were overcome by 20 hp (aerodynamic drag, transmission drag, final drive drag, tire drag, a little "dragging brake" drag, etc.) that are now helping the car to slow down. If the wind is a headwind the car will decelerate faster than if the wind is a tailwind. If it is cold out, the air is denser, so the car will slow faster. If the tires are cold and stiff their hysteresis is higher, so more energy is extracted than if they are warm.

How much of the slowing is caused by the engine? Very hard to separate out, even if you had the consistency of a chassis dyno inside a building. How much of an increment between a reading from last year and a reading today is cause by engine wear??? How much is caused by wind, temperature, tire pressure, tire tread thickness, etc., etc.

Separating this out even with a really good, repeatable, accurate accelerometer: essentially impossible.

A simple compression test will give you a far better, more useful indication of engine condition than either acceleration or deceleration rates. If you were to measure something with a cheap accelerometer like those in a cell phone, then acceleration would tell you more than deceleration, because the signal-to-noise ratio is better (because the signal is bigger.) A stop watch on a consistent stretch of road would provide better accuracy.

Old engines continue to produce relatively high levels of power even when almost worn out. Most general aviation engine at 2000 hours (overhaul time; 300,000 miles through the air at fairly high power outputs) run about like they do when new. When they produce noticeably lower power than normal, you've let things go too long.

Some things to try:

- Pull the rope on and old lawnmower vs a new one.

- Measure the differences in motoring torque required to spin a close throttle engine in an engine lab. Test at 100 miles, 20,000 50,000, 100,000 and 200,000. My guess would be that torque would reach a minimum at 100,000 miles, and then be a tiny bit higher when the engine gets into oil burner status.

Thanks for the reply- just to pick up on a couple of points, I'm not chasing test laboratory accuracy- I'm more interested in establishing where the rolling losses are. What I would be interested in doing is, in the first instance, measuring the rolling resistance. Stage 1-Find a quiet, clear, straight, flat stretch of road, accelerate to cruising speed, dip the clutch to get the engine out of the picture and log the deceleration, whether with the accelerometers on a smartphone, a gps log, or a stopwatch. Repeat in the opposite direction to average out wind effects. If I can get repeatable, consistent deceleration profiles, then let the tweaking begin- roof rack, tyres and tyre pressures, and move onto Stage 2- cut the ignition at cruising speed and decelerate with the throttle wide open, and hopefully get an idea of the engine drag.

A simple compression test will give you a far better, more useful indication of engine condition...
In my experience, I'd have limited faith in cranking compression tests, firstly because of the variation in battery and starter motor condition, and secondly because of poor standardisation of test procedures- any attempts to compare notes usually elicit replies of: Did you take out all the plugs?/Did you open the throttle wide?/Did you let it do just 5 compression strokes?/Did you stand on one leg and whistle "Sweet Home Alabama"? I'd have more faith in a leak-down test.
Pull the rope on and old lawnmower vs a new one.
Very probably misleading- I'm trying to get clear on the processes at engine running speeds.

the thottle valve closes and dose'nt allow air to enter the combustion chambers. these's no fuel to ignite so the engine acts as a vaccum pump. it's the reverse of coasting.

Normal Diesel = no throttle.

I personally have never ever heard of a diesel with a butterfly valve, (neither has Wiki, read here:- http://en.wikipedia.org/wiki/Manifold_vacuum ), but that is not to say that somewhere there is one, (but it seriously reduces engine efficiency according to that Wiki link)....but I would certainly think that its not usual at all nowadays....perhaps an expert in such matters can chime in with a few selected web links.

I posted a link some time ago in this blog, that demonstrated that Diesels with a Turbo (most car/truck Diesels nowadays) have an interesting effect in that due to the fact that the Turbo is blown around by the exhaust gases, so when there are little or no exhaust gases (no Diesel is being pumped into the engine as no power is required), the Turbo runs extremely slow, or even possibly stops, effectively blocking most air from entering the engine and turning it into a huge sort of vacuum pump.

Slowing the engine/car down quite dramatically.

You notice the effect especially good when going downhill in a low gear, in spite of say having a heavy trailer really pushing you on, the engine makes quite a completely different and quite loud noise to normal....I have heard it many, many times as I have been driving Turbo Diesels in cars and small trucks since around 1988.

I can also say that in spite of driving with heavy trailers many times, I have never had to continuously brake when going down a steep hill, there was always a gear that kept the speed down to what I wanted, though sometimes I would need to use the throttle to speed it up a bit, as otherwise it would have been too slow for me, and the next gear up did not slow up the whole car/trailer unit enough....e.g. the car would accelerate slightly. So speed would be then throttle controlled and not brake controlled.

Me being a safe sort of person, I don't accept any acceleration in such situations that I cannot control with just the throttle. I like to keep my brakes cold for emergencies only....but basically its never been needed....

I am sure that many here have experienced exactly the same as I have when driving a Turbo diesel, happily accepted it, but been unable to describe why exactly it happens....Wiki helped me to understand why, though not the link above, the previous one I posted.

On old tractors and trucks with an in-line diesel pump the rack position was determined by a vacuum "govenor" [Diafram and spring]. The vacuum was changed by the throttle valve in the intake. With high vacuum the rack is pulled towards the stop position against spring action.With engine stopped, the spring push the rack to max feul delivery position. Most that I worked on were CAV pumps.

With diesel engines, as most are, with no throttle valve, the cylinders are almost filled with air on intake stroke [very little vacuum] and with a compression ratio of about 20:1 compression takes a lot of power...braking the vehicle.

Maybe on a single cylinder diesel, but any engine with two or more cylinders will not show much engine braking at all as the efforts are distributed across all cylinders. This has been discussed earlier in this blog, start reading at the top..

That is the problem we are discussing....

Only petrol engines with a throttle or diesels with a Turbo demonstrate such braking. Most normally aspirated diesels show little braking effect, just engine resistance to turn.....unless its a single cylinder Diesel of course.....

We do have single cylinder diesels here:-

http://www.youtube.com/watch?v=tr_b6mTnHEo

http://www.youtube.com/watch?v=1i6Docas9Eo

You are right Andy; Thanks

Jurie

I drive a 4 cylinder non turbo diesel Ford Ranger and there is engine braking. Less than a turbo but still there including your observation on having to apply throttle when descending lest I slow down to much.

Pumping air through the valves dumps energy.

Most of the braking is through pumping losses through the intake and exhaust valves.

An unfuelled compression stroke/power stroke is elastic and provides little braking due to cylinder wall heating. An engine with crap compression, brakes even better with air squirting past the rings....

Jacobs brake opens the valves on the compression and power stroke (or is it just the compression stroke) to provide even more pumping losses.

It dumps the pressure at or very near top dead center after the energy has been used to compress the air charge giving nothing back on the power stroke when the Jake brake is used.

Thanks for that.