Making an Older Engine with Disengageable 'Engine Braking'?

Personally, I would prefer to come up with a way to do this on a SOHC with tappets (inlet & exhaust on same side), but this is looking very squashed.

The idea is to hold the valves open as far as possible, but without (of course) risking them hitting the piston at TDC. I'm ignoring the possibility of holding the exhaust valves open as well as this would cool the exhaust too quickly and cause pulsation issues.

I can picture a method where the cam gear is disengaged, but the oppotunity to permantly throw out the timing (& even wrech the engine) seems too risky.

what are you talking about ?

What causes engine braking? - pumping losses mostly.

I want a modification that can take away the resistance of turning an engine over when it's not making power - like the resistance of cold cranking... So, valves held open = no pumping losses.

G'day,

Just as a point of information, GMH's variable engines close both inlet and exhaust valves when disengaging four cylinders (of 8). The losses on compression are balanced by the gains due to expansion after TDC. Net loss? about zero.

Best Regards,

Royce R. Vines

"In a time of drastic change it is the learners who inherit the future. The learned usually find themselves equipped to live in a world that no longer exists." - Eric Hoffer"

Thanks for the info. I have heard of this approach also - using the principle of a gas-spring I think. There must still be higher losses here than a totally open 'route' for air in & out, and the fact that 4 cylinders are still active changes the balance too I would think.

Be careful most modern engines don't have mutch clearence between the top of the piston & the valve head hence the cost when a timing belt snaps & all the valves that are open hit the pistons and bend, So be sure to check how far you can depress the valve without hitting the piston.

Bazzer

Cheers Bazzer.

I wd be going with a '60s engine and I'm sure most of those are less tight. I;m still not sure if a broken chain will wreck it though (?).

If the engine has enough clearance for between valves and piston, what difference will a broken chain make????? Other than the obvious of course!!

I doubt that.....

I am sure the big companies thought that through carefully first.....less loss with closed valves is my personal bet!!!

What are you attempting to accomplish?

How about modifying the head(s) to have a compression release valve in them? Port the output from the compression relief into the exhaust...probably after the cat.

ah ha read your first post again you want an engine that gives power then stops then power then stops etc with a valve lifter so there is no pumping losses.

It wont work there is no valve clearance if the valve are are down when the piston comes up they will be hit.

also when piston goes down the will be no suction as well as no pumping no suction means there will be no new fuel/air mix brought into engine for next power cycle.

you idea is good but totally impracticable.

a better method would be to have the engine run at its most economical speed and disengage the clutch periodically but i see no useful purpose in this either

Because when the engine is disengaged the vehicle will slow down

so when the engine is re engaged the engine will have to work harder to bring the vehicle speed back up

it would be more economical to keep engine running at constant speed and leaving it connected all the time unless you want to stop of course

"it would be more economical to keep engine running at constant speed and leaving it connected all the time unless you want to stop of course"

This seems counter to what the 'pulse and glide' guys reckon - of course they may be wrong or just over-exaggerating!

But, there is good hard evidence that an engine is most efficient under about 80% load at medium to higher revs; and that idling is very thermally inefficient.

on the flat pulse and glide may work.

going downhill engine braking is needed unless you are going to use the brakes

going uphill pulse and glide would not work at all

but good luck on your efforts

Most auto tranny's do not supply engine braking in drive.

But, there is good hard evidence that an engine is most efficient under about 80% load at medium to higher revs;

This is pretty close to correct. In general a simplified Specific Fuel Consumption curve looks like the torque curve, upside down. The torque curve, however, is based on full load (full throttle) conditions. When you look at the map of specific consumption vs both load and rpm, low rpm is favored. For fuel efficiency alone, you are better off lugging the engine: this results in large throttle openings for a given power output, and thus fewer pumping losses.

Sorry, but an engine is at its most efficient when it produces its max Brake Mean Effective Pressure (bmep). This occurs at max torque, whatever the rpm. Some engines get max torque at low revs, others (usually high performance and/or EFI motors) at high revs. However, EFI engines usually produce about 90% of max torque from 2000 rpm up to max toque rpm ( up tp 4500)

Sorry, but an engine is at its most efficient when it produces its max Brake Mean Effective Pressure (bmep).

Wrong, but a common misconception. In simplified terms, what you say is roughly correct -- that's why I mentioned that the full load BSFC curve looks like the full load torque curve inverted. That has little to do with real driving, however: people do not drive with the throttle full open all the time. To see the efficiency in real driving, you must look at the BSFC at various loads and rpms.

Here's the map for a Saturn:

As you can see, best BSFC is at a point less than full load.

In the chart below, I added a line that represents 20 HP, about that required by a Saturn for cruising on a level highway.

As you can see, the line gets closest to the 250 g/kWh boundary at about 1300 rpm. So to cruise most efficiently, this car should be geared for 1300 rpm at highway speeds.

Now you start getting into the rather impractical side of engine theory. I maintain that to drive with the best fuel consumption, keep the engine at approx max torque, which in modern engines is usually anywhere from 1500-2000 rpm and up. Lugging an engine, apart from not being too healthy for the engines longevity is not the most economical method of driving due to various road and traffic conditions, unless you have a very efficient dual clutch DSG ( an oxymoron?).

economy

http://www.google.co.uk/search?q=acheiving+fuel+economy+on+modern+petrol+engines&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7GGLT_en&redir_esc=&ei=uDOsTOOwL5WSjAfVs5DMCw

So you want to remove the compression from the engine, on demand, without modifying the engine. As posted above there are problems with holding the valves open. Current engine management systems are leaving them closed.

With the current 10MM spark plugs that are now available, you could make up adaptors that would fit between the original spark plug hole,(14MM) and the smaller 10MM plug. Connected to these adaptors could be the compression release valves.

IMHO, any gains from this system could be easily made up by using a newer, more efficient engine design.

If you want to improve efficiency of the ICE design, try to work with solenoid controlled intake and exhaust valves. From there, any valve timing, duration, and compression release becomes possible. Good luck.

Hi thought about your request the only easy way i would try would be to shut off the inlet this way there would be a partial vacume in the cylinders which would assist with the piston stroke which would would work in you favor

the flap could be a recycle exhaust brake as fitted to some commerical vehicles

just remove it from the exhaust and place it after the carb on inlet manifold

Only problem is the may be a bit of a hiccup when flap is opened

See #4

It looks like you may have things a bit backwards. To avoid pumping losses, you should try to avoid pumping. That means leaving the valves closed, not open. This is what is done in engines that have variable displacement (running on 3, 4 or 6 cylinders, for example). In this way, the energy used to compress air in the cylinder is returned as the air expands, like a spring.

A few decades ago, compression releases were used on dirt bikes to provide braking effect. These vented the combustion chamber through a small valve. Intake would occur (with its losses). Then compression would occur, but the pressure rise would be small because of the venting. The energy required to force the air past the compression release valve would be extracted from the bike's kinetic energy. Then the "power" stroke would occur... but there would be no power, because there would be no mixture to ignite. Some small amount of inflow would occur through the compression release valve, during the "power" stoke. (Compression releases worked a little like Jake brakes on a truck.)

The variable displacement systems achieve an effect vaguely like "pulse and glide", but more smoothly. The idea in pulse and glide is to disengage the clutch during the glide, so that the car does not suffer compression braking. (Closing the valves would have a similar, although less pronounced effect, because there are still frictional losses from the engine). While the pulse is occurring, the engine is running under moderately high load, where it is more efficient than under light load (where it would otherwise be while driving at a constant speed). In a variable displacement engine, the operating cylinders are operating under high load, making them efficient (even though the engine as a whole is producing low output) and the other cylinders are just along for the ride (with, on average, one cylinder under vacuum roughly balancing another under compression).

Motorcycle compression releases are environmentally unfriendly, ejecting a lot of unburned mixture directly into the air. In variable displacement engines, injection is shut off in those cylinders that are not operating under load. The systems are incredibly complex, and require fast computing to operate smoothly. If you are going to deal with all the complexity, then you might as well produce a real hybrid, like a Prius. This provides much higher efficiency, particularly when combined with the Prius Atkinson cycle engine.

Your idea has merit. When your idea is implemented in a way that leaves imperceptibly small times between power strokes, you end up with a variable displacement engine. Shutting off all the cylinders even for two out of four revolutions (let alone 3 out of 4) produces too much vibration, whereas shutting off some of the cylinders provides for smaller, more frequent power pulses.

In my view, a Civic, which is comparatively simple (and inherently efficient because of its smallish size, light weight, and small engine) is more fun to drive, uses less resources in its construction, and uses less fuel than the bloated Accord V6 with is variable displacement system. If they were both the same price, I'd opt for the Civic. Plenty of people prefer bloat, however, so variable displacement systems will survive.

Thanks for the 10mm plug in a 14mm hole idea Bob - I like! Thanks also to MB for an informative few paragraphs! To Peter, I think I've given the impression of a closed throttle reading back your exhaust valve suggestions, in fact I meant WOT.

NTL I'm converted (halleluya!)! by going for a low in-cylinder pressure with 'all valves closed' conditions - it does seem better - if it can be simple and smooth enough.

I would add though that when I was originally suggesting an inlet-always-open condition, I would be running WOT - Sure air will be whizzing in a out of the cylinders, but I would have thought that the work involved would be only a little more of a drag than bouncing the gas-springs. ....... I do like the suggestion of the late early closing inlet valves (or an equivalent effect) in the no-air-in-or-out design, where the low pressure spring is balancing the momentum of the engine and drive train (have I got that right?).

For the gas-spring mode, I might suggest that fitting dog-clutch on the cam sprocket would allow the re-engaged cam to pick up again in the right place. In theory the whole cam could move away from the head (oops all the shims have fallen out!) and the poppet valves would 'pop' shut against the springs.... I don;t know?!

To clarify: Definitely no fuel would be pump to the engine during such operation (fuel injection -ideally multipoint- will be a mod on the engine before any of this p*ssing around starts anyway).

exhaust brakes

http://www.google.co.uk/search?q=exhaust+brakes&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7GGLT_en&redir_esc=&ei=MVarTOGSDZGUjAe-pYzdBw

using the same idea but on the throttle

There are some direct injection engines that are very fuel efficient as built. They also have the luxury of adjustable cam timing. If displacement on demand is also used on one of these engines, you would only have to "teach" the engine management system to do some of the things that it already does, ad different times. That could be done with a program writer.

Why bother with open or closed valves and the inherent mechanism to make it happen? Why not use something like the freewheeling on the older overdrive transmissions?

Like the P4 Rovers used to have? -That operate if the car wheels are going faster than the engine speed-

There would be an even worse jolt when the system is moved into the locked position by the Engine Management - the aim of my system cut off fuel at short intermittent periods and rely on carrying the momentum of the flywheel <plus rolling wheels/drivetrain> but against far less pumping losses (gas-spring in pistons).

It could sort of work with a free-wheeler but the engine would stop almost straight away in the 'glide' mode and would need restarting for the 'pulse' giving repeated heavy loads on the starter & battery.