So I got to thinking, (dangerous I know) was thinking and researching ideal compression ratios(CR) and came across a bunch of research that said 14.8 CR was the ideal for power and efficiency. Whether or not I believe that or not it got me to wondering how in the heck I would start an IDI with such a low compression ratio. Sure I could add a sector heater or 5 but what about a way to temporarily increase the compression, valve opening timing changes, a supercharger that builds pressure at idle, then it hit me.

Backpressure, a valve to block the exhaust enough to build a little backpressure would increase effective compression pressures and temperatures enough to start any diesel that the starter could turn. The backpressure wouldn't have to be very high. Just 6.6psi of back pressure would make a 14.8 CR engine have the same compression pressure as a stock NA 21.5 CR engine. All it would take is a spring loaded valve that takes a certain amount of pressure to open like an exhaust pressure regulator and then is opened permanently by heat, turbo pressure, or the vacuum pump or something.

My brain seems to think this has been done before, what does FTE have to say?

 

low compression ratios are for direct injected turbocharged engines.
when Dave Sprongle built his engine he cut compression down to around 17:1 so he could add more boost, and had a bear of a time cold starting it.
i can't even imagine trying to start a diesel with only 14.8:1 compression.
i have seen gas engines with ratios that high.

 

My truck is sitting right at 18:1, and that's static, cam also bleeds a tad more, and It starts fine in freezing conditions. It does have a slight lope for a few minutes, but fires right up with no issue.

Leo, that's an interesting idea, and would help warm the truck up faster, the 7.3PSD uses a similar tactic for emissions, the turbo has a flapper that closes when its cold to bring back pressure up.

 

Thats what our 5 ton trucks did in my artillery unit, the exhaust brake had 2 settings, one for auto-retard one for warm up phase. On a cold engine, hit the warm up setting and it closed the exhaust brake a bit to help warm up the cylinders.

 

I just realized it wouldn't work for starting, the intake manifold would have to have the same pressure. Duhhh as soon as the intake valve opens pressure between the intake manifold and cylinder equalize.

It would work to help with warm up, but not the first light off that gets things going unless there was a positive displacement superchager.

............

I am still curious about this low 14.8 CR. I came accross a paper on a study/experiment on optimizing the compression ratio. http://www.arpnjournals.com/jeas/res...as_0408_88.pdf

Quote:

In order to find out optimum compression ratio experiments were carried out on a single cylinder four stroke variable compression ratio diesel engine. Tests were carried out at compression ratios of 13.2, 13.9, 14.8, 15.7, 16.9, 18.1 and 20.2. Results showed a significant improved performance and emission characteristics at a compression ratio 14.8. The compression ratios lesser than 14.8 and greater than 14.8 showed a drop in break thermal efficiency, rise in fuel consumption along with increased smoke densities.

So like Justin said he's 18:1 static with more bleed off from the cam and doing well so far. Justin how low you think your dynamic CR is when taking the cam into consideration?

This of course got me to thinking about Miller cycle(leaving the intake valve open later to bleed of compression). Come to find it's used more then I thought, even on diesels. Couldn't find any info on light to medium automotive diesels yet, but I'm sure it's out there, however in locomotive diesels it's common apparently and some trucking(OTR) diesels, the Cat ACERT engine is apparently a Miller.

I did some math a while back and figured with a 7.3 with 6.9 heads and some other tweaking 24:1 was possible, and. If something as low as 14.8 creates more efficiency which I can only assume comes from lowering pumping and heat losses by not compressing anything more then is needed for clean ignition. Then why not a Miller cycle with 24:1 static/expansion ratio and 14.8:1 dynamic compression ratio? Sounds all up side to me, lower EGTs, more fuel efficient, more power, head gaskets stay together, etc.

Of course more has to be looked into on the 14.8:1 dynamic ratio, sure sounds low to me and that's only one not so great study. But seems quite a few people out there are doing as Justin and lowering the CR both static and dynamic.

 

I think there's something slightly flawed with the paper:
If you look at power, the engine is rated for 3.75KW. They only tested it at 3KW. They also don't specify the RPM.
I would want to see a series of tests showing maxmum HP/KW at varying speeds, for several compression rations.

Also, I'm guessing this was a NA engine. If so, yes, reducing the CR to the point of clean burn of the fuel you're putting in will give you the best result.

This, however, only applies at part loading conditions... If you want more power out, you need more fuel and air. This means more effective CR.

You can do this by increasing the compression ratio itself, or by increasing the pressure of the air via a turbo- or super- charger.

I suspect, had they tested the engine at >3KW(i.e. at the engine rating or beyond), things might have been different.

Also, varying speeds(needed in any mobile application) would have a large impact on things.

 

Yeah I have no confidence in that study, it's very lacking in details. That said anecdotal evidence from other builds seems to indicate that if you can get it started reliably then something in the area of 15-16 dynamic does quite well.

It would of course have to be charged in some way to make good power, but this is where it would shine. Put just 6 PSI in it and you have the same compression pressure as a stock NA engine without taking all that energy and heat loss to compress it in the cylinder. Compressing it in the turbo or even supercharger which is a more efficient way off compressing the air.

 

You *could* look at something like this:
http://en.wikipedia.org/wiki/Hybrid_turbocharger
Though to be honest, I wonder about taking a turbocharger and hooking it directly to an electric motor of the sort used there. Now, you use electricity to power it and give it a boost when starting/accelerating, exhaust gasses power it most of the time(motor freewheeling), and you could use the motor to put a load on it, turning it into an exhaust brake.

 

My understanding at this point is that in spite of some major development money from OEMs that tech like that has never really worked. The motor it takes to drive a changer is too big with too much draw to make it worth it or for exhaust to be able to spin it, but idk I'm just pulling this out of my ****.

Although very expensive IMO the ideal would be a positive displacement supercharger pushing into a turbo. The supercharger maintains a constant pressure for idle, low throttle, and throttle response. When the turbo spools it pulls through the supercharger reducing or eliminating the load on the charger.

 

FWIW a much better study http://www.diva-portal.org/smash/get...FULLTEXT01.pdf