Tuesday, October 20, 2015

Continental GT to test whether new head design works

Shawn Chriswell's Royal Enfield Continental GT under testing
in stock condition. Tank is taped to prevent errors in mileage.
Part 3

A Royal Enfield Continental GT will test whether a patented new cylinder head design brings better fuel mileage and reduced emissions.

A grant from the National Science Foundation will help pay for the test. But inventor Shawn Chriswell notes that proving the cylinder head works won't nail down why it works.

"I know it works. I've tested it. But why it works — these are theories right now. I don't have the multi-million-dollar University of Michigan to (figure) it out."

He explained his theory to me on the phone and sent the drawings below to illustrate what he thinks is happening.

The cylinder head design was created by Shawn and his father, the late Darrell Chriswell, in their Longmont, Colo. shop. Shawn used hints picked up from a flow bench they had designed.

Shawn emphasized to me that he is not trying to sell the Royal Enfield community cylinder heads. He just likes the Continental GT, so he chose it as a final test bed. (Prototypes of the cylinder head design have run on his Harley-Davidson V-twin.)

Shawn Chriswell's prototype front cylinder head for a Harley-Davidson.
Piggy-back cams require tall cylinder head cover at right;
spacer at bottom prevents piston contact; compression stays stock.
His cylinder head features a ridge across the combustion chamber. The big intake valve is on one side and two exhaust valves are on the other side of that ridge. The valve stems of the intake and exhaust valves criss-cross.

This apparently results in better use of the fuel mixture. That is suggested by the improved miles per gallon Shawn recorded with prototype heads fitted to a Harley-Davidson.

Why would this be so? Here's how I understand his theories:

I tend to think of the fuel mixture entering the combustion chamber evenly — flowing down the tapered shape of the intake valve stem and around the valve face — pretty as a picture.

But in fact the mixture wants to get in there as fast as it can and it takes the shortest route it can to follow the piston as it drops in the bore.

Shawn says the mixture naturally wants to use the lowest part of the valve opening.

In his design, the lowest part of the intake valve puts the mixture close to the center of the combustion chamber.

Since it opens at the angle it does, his valve in effect "gets out of the way" of the entering gases.

The spark plug is in the half of the combustion chamber that includes the intake valve. The mixture ignites there first, spreading to the second half of the chamber where a second flame front is created. The first flame front may act as a short of shaped explosive charge, energizing the second half of the combustion chamber.

"It's just developing a better 'pop,'" was the way Shawn put it.

Together, the two flame fronts rebound when the piston reaches the bottom of the stroke. The twin exhaust valves get out of the way of the exhaust gas as they open, giving the spent charge the shortest possible route out of the motor — for awhile.

The rising piston and the ridge across the combustion chamber combine to cut off the scavenging effect of the relative vacuum that forms on the other end of the exhaust valves.

I'm used to thinking that it doesn't matter if a little of the fresh gas coming in during valve overlap goes out the exhaust with the bad gas, as long as the bad charge is pulled out fast. Shawn had to explain to me why reduced scavenging is a good thing.

"...As the (exhaust) valves are closing, piston moving up, I think the intake side of my divided combustion chamber will start to fill with fresh intake fuel air mixture. The fresh charge is contained in the intake side of the combustion chamber as the exhaust is pulling out more burnt gases from the exhaust side of the combustion chamber. I think that this reduced scavenging of the fresh intake charge helps maintain a higher percentage of fuel air/mixture in the cylinder vs. NOx, CO, SOx, C02. We are speaking of the minutest amount  of air fuel mixture. But at 3,500 rpm it adds up."

Take at look at Shawn's drawings below for an idea of how it all works.

Running the criss-cross valve train would be "no problem" with double overhead cams, Shawn says. But, for the NSF test, not only must the Royal Enfield retain its push rods, but the new valve train has to be a bolt-on modification.

So the new cylinder head design uses "piggy-back" rockers, one on top of the other instead of side-by-side. This fits under the stock tank, "although it's close," Shawn says.

More importantly, Shawn doesn't think the valve train arrangement will cancel out the gains from improved combustion.

He's very close to testing the new head on his 2014 Royal Enfield Continental GT, he said. When it's tuned and ready, the bike will be tested at a Denver facility as stock, with stock catalytic converter, to establish a baseline. Shawn will then attach his new cylinder head for testing.

Part 1 — Royal Enfield will test cylinder head design

Part 2 — How the project started

NEXT: Part 4 — The results (when available)

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