Variable compression engine

[Lazy JW]

There was talk here recently about improving combustion efficiency by using a means of variable compression. Here is a primitive method I came up with. Probably has been done before; there are many things wrong with this design but I hope to stimulate thought and discussion.
Joe

 

[Silver Shadow]

Wouldn't the compresion relaese petcocks on the old John Deeres be simpler? One could time them mit the computer.

 

[Lazy JW]

My intention is to maximize compression at part-throttle to get equal power out of less fuel. Blowing compression out of the petcocks will waste fuel and energy.
Joe

 

[HUMINA]

Saab developed a variable compression engine. I don't think that it has made itinto a production vehicle though. They essentially rotate the entire head realtive to to the crank.

http://www.saab.co.uk/main/GLOBAL/en/vepsilon/index.html

I remember first hearing about it years ago (around 2000). But a quick look at the Saab website and it doesn't look like it ever made it to the street.

 

[fb71]

no, it never made it to production. Check patents with Porsche, they had one very similar they experimented with. They also patented a "scissor" piston engine.

Look here as well
http://en.wikipedia.org/wiki/Opposed_piston_engine

 

[rbohm]

8) personally i would rather see you figure a way to vary the lobe separation angles rather than vary the compression. this will do the same thing, in maximizing the power output at low speeds, as well as maximizing the power output at higher speeds as well. and it might be easier.

 

[fb71]

some big trucks and a BMW use hydraulically actuated valves, with infinately varible lift, duration and overlap. This was actually pioneered by Preston Tucker on the Franklin boxer six cylinder engine he used in the Torpedo. It didn't produce enough horsepower and wasn't very driveable, due to the imprecise valve control thru modified diesel fuel injector pumps. So the hydraulic valves were dropped in favor of traditional camshaft actuated valvetrain. That version of the engine developed well over 115bhp, vs about 88bhp.

 

[Pinhead]

... valves, with infinately varible lift, duration and overlap.

Hmm... Sounds familiar...

Technically current gasoline engines are variable compression ratio engines, variable dynamic compression.

Larry Widmer builds his engines with as small of a combustion chamber as possible, producing CR's of 23:1 and higher on gasoline... The dynamic compression is lowered through static valve timing, with a turbo/supercharger making up for top-end.

http://mpgresearch.com/viewtopic.php?t= ... =soft+head
http://theoldone.com/articles/The%5FSoft%5FHead%5F1999/

 

[Lazy JW]

...

Technically current gasoline engines are variable compression ratio engines, variable dynamic compression...

Correct. I was trying to come up with a means to vary the static compression ratio, plus capture some of the energy that is lost through the cylinder head.

I am also hoping someone will devise a better way of accomplishing this than my feeble efforts. Keep those brain cells working!
Joe

 

[Anonymous]

It gets to be more mechanical complexity than I'd want, tho' I could be talked into it, but how about a modification of the old sleeve-valve engines to alter compression, intake/exhaust events, the whole deal? That's a lot of parts getting slung around, but it might work well on an engine you didn't have to wind up tight to make power. Some of the last and most impressive English aero engines had sleeve-valves (Napier Sabre, Rolls-Royce Eagle, Bristol Centaurus radial), as did some earlier American auto engines (Knight, maybe Franklin). Some advantages of sleeve-valve engines is that they are relatively compact, and that they have can have good airflow charactoristics since they don't have the head of a poppet valve to flow around.

 

[fb71]

Pinhead,

Those "PowreValvz" or whatever they're called, are not infinately variable nor precisely controllable. Neither is VTEC, VTTi, VCT or any other camshaft actuacted valve. But, they (PowreValvz) are an interesting device, albiet one of questionable design/durability/application.

What I mean, is a valve that can be opened exactly when I want, for how long I want, and as much as I want ("I" being the computer).

And yes, I too am fascinated by the Widmer engines!

And I like Smitty's ideas too. I think some Franklins did have sleeve valves (and Franklin was championed by aviation hero Charles Lindberg). Ever play with any old Yamaha rotary valve engines? The only problem I can concieve (and have experienced) is sealing of those types of valves.

 

[Lazy JW]

It gets to be more mechanical complexity than I'd want, tho' I could be talked into it, but how about a modification of the old sleeve-valve engines to alter compression, intake/exhaust events, the whole deal? That's a lot of parts getting slung around, but it might work well on an engine you didn't have to wind up tight to make power. Some of the last and most impressive English aero engines had sleeve-valves (Napier Sabre, Rolls-Royce Eagle, Bristol Centaurus radial), as did some earlier American auto engines (Knight, maybe Franklin). Some advantages of sleeve-valve engines is that they are relatively compact, and that they have can have good airflow charactoristics since they don't have the head of a poppet valve to flow around.

I thought about sleeve valves; modern CNC machining technology might even make them practical on a mass-produced engine. Note I didn't draw any valves in my hypothetical engine, leaving that aspect open for thought. My design would definitely be a low-speed affair with all that weight thrashing around.
Joe

 

[lyonsy]

cat the only company i know employing the veriable valve tech is not really veriable valve.
and its not hydrolic valved ether its still conventonal with cam rockers valves etc but essientialy has a second jake brake fitted to the inlet valves
all it can do is hold the inlet valve open longer to aloow more air in as it operates at over 60psi.
this is to allow better scvanging and to make sure they get a 100% air fuel mix with no previous burn in the cylinder.
basicly the same as miller cycle engine

you could probly do your idea on a commo knoocker but its a 2 stroke so some engergy will be lost.

if you want to make the cylinder volume change id say use 2 pistons 1 as a head the other as a normal pistons.
when compression can be increased the head pistons slides down the bore reducing volume but increasing comp.
i can only see this working with a side valve design but.

a puematic valve system whould proably work better though with direct injection build the engine with 30.:0 comp and let valves bleed off the cyl pressureout the exhaust but with no fuel its not going to kill your emissions or other problems.

 

[xctasy]

That's fine stuff above, showing some reall initiative. I think most everyone fails to understand that reworking the axe is the only way to get the go forward. A whole new engine creates decades to perfect, so revolutionary opposed, swashplate or scotch yoke engines, or barrel valve or desmondric yadda yadda yadda, they are all alternative schedules which haven't had the fortune of 150 years of developement.

It is, in my opinion, easier to have an adjustable piston dome, and you can range the CR from about 8:1 to 11:1 or so. A negative 20 cc dish to a postive 10 cc dome should work, and give that compression ratio change. Spring load it to become a dish under high pressure, and a dome under low pressure

The piston is a large block of alloy which could be like a spring loaded C4 type servo which gives 8:1 under 14 pounds or supercharger boost, and 11:1 under no boost, all defined by an internal spring. The centre of a 200 cube log engine piston is a 6.5 cc dish, which could be modified in a forged piston to behave as a variable trench.

The ohv cam layout can be reworked like the MGF 1.8 VVT engine. There, a built up cam can be phased by sequence to vary overlap. The rocker arm could have a variable pivot, and the valve lift could be varied infinately.

People haven't gotten there heads around integrational engineering. Thats where you get some old iron engine block, and whack a finely optimised part on it, and improve its efficency. If you can make it work, well and good. If you can then build it down to a price, and still get the mesurable gains, then the design is a sure fire.

 

[rmt]

I have had several variable compression engines over the years. 0-20% compression in one cylinder, 50-75% in a few others, maybe 85% in a couple. Though these engines did work, none of them worked very well. Pretty much a waste of time to develop the concept any further, so I'd just fix them and put them back together the non-variable way.

 

[addo]

Why not a more subtle method of varying compression; rather than a lever this uses a screw setup:

http://www.antiquemodeler.org/Engines/diesels.htm

 

[Lazy JW]

I was hoping you would chime in here Deano.

...People haven't gotten there heads around integrational engineering. Thats where you get some old iron engine block, and whack a finely optimised part on it, and improve its efficency...

Trouble is, most folks have spent their efforts trying to improve the PUMPING efficiency of engines in order to make more power per cubic inch. That is a fine thing, and they have been quite successful in their endeavors.

What I'm aiming at is getting folks to think about THERMAL efficiency so as to extract more power per unit of fuel. I believe this is possible; also necessary given our limited amount of natural resources. If as much effort were put into thermal efficiency as has been put into pumping efficiency we would likely be a LONG way ahead of where we now are.

As mentioned above, standard gasoline engines are indeed variable compression, like maybe zero compression at idle. How many people even think about that little detail? Check out my little drawings above, count the squares and tell me how much compression that engine has


I thought about side valves but don't like the idea. I was hoping someone would throw in some practical sperical rotary valves made of unobtanium or something really razzoo. Of course a two-stroke would work but can they burn clean enough?

Keep the thoughts coming,
Joe

 

[Anonymous]

A couple years back, I came up with an idea to internally balance a radial engine. My idea was to make a 5 cylinder radial engine that was small enough to fit into a vehicle,, where two banks could be stacked together to create a 10 cylinder for high power applications. Radials are not naturally balanceable in their current design, so in order to meet NVH requirements for automobiles,, it would have to be balanced.

Anyway, one of the consequences of the mechanism I came up with to balance the radial also permits you to adjust the piston to tdc clearance on the fly,,, thereby adjusting your compression ratio.

Additionally, the valves on a radial engine are driven by cam rings. if you separated the exhaust and intake lobes onto separate rings, you could essentially adjust both intake and valve timing and LCA on the fly as well.

Unfortunately, I can't show you the mechanism for the variable compression ratio because I haven't completed the patent process yet. I promise I'm not just blowing smoke though.

You can, however, look on the net for how the cam is driven on a radial, and you can probably picture how the intake and exhaust could be separated onto two rings,, and then mechanically driven to create variable valve timing.

It's a pretty radical idea, but surprisingly simple. You'd have to be savvy in control systems engineering to design controllers for this, but lucky for me,, I'm getting my masters in Controls as we speak .

 

[Lazy JW]

Somewhere in my archives (fancy name for junkpiles) I have an old Popular Science magazine that has an article about some large radials built for stationary use as power generators. They had a mechanism that eliminated the master connecting rod and all of the rods were equal with equal orbits around the crank. IIRC they were "Nordhoff's"? Not sure about the name though. They were installed in a power plant in Texas and fueled with natural gas. I believe the design was balanced.
Joe

 

[Anonymous]

Joe, you mention spherical rotary valves as a possibility. Did you see the occasional references here to the Coates heads? Most of the reaction here was skeptical, mostly about long-term reliability as I recall, but with WATER INJECTION, my favorite compression-enabler, maybe carbon-scoring would not be an issue.

Jim, disc rotary valves on two-strokes work better than barrel rotary valves, but how would you apply them to four-stroke auto engines?

Some of the current generation direct-injected, pressure-oiled 2-stroke outboards are quite clean, fairly fuel-efficient, and get more power out of a smaller, lighter, cheaper-to-manufacture package than comparable 4-strokes. Adjustable exhaust timing appeared on racing 2-strokes in a big way in the early 1980's. I'm not promoting them as an answer to any automotive problem, but it's worth keeping an eye on.