I was reading on THIS and it says in the second bullet there about the "Nuts and Bolts".......

The compression ratio of the pistons and the timing must go together. High compression pistons belong only in gasoline engines that are naturally aspirated. Turbocharged diesel engines must have low compression pistons in order to produce high horsepower. The lower the compression ratio the less wear on rod bearings and the lower the internal pressure will be when the injector fires.


Now my question is...Is that true or did they do a mis print?I thought that we need high compression pistons and gassers need low compression cause we run 18.5:1 and they run 12.5:1 on AVG.

thank you for clearing my confusion

 

This is my understanding:

During the compression stroke, the temp of the incoming charge rises. In a gas engine, this incoming charge is gasoline and air, mixed, and if compressed too much, the mixture explodes before the spark goes off. This spike in pressure will damage the engine if sustained or severe.

In a diesel, the incoming charge that gets compressed is air, and the heat that is created by compression is what causes ignition when the fuel is injected.

High compression pistons in a gas engine are usually coupled with longer duration cams, which both moves the power peak to a higher RPM, and lowers the low RPM compression pressure... For the same compression ratio, an engine with short duration cam (IE stock) is more likely to have octane ping, where a performance (long duration) cam may be more likely to be able to run on pump gas without knock/ping...

The more gas and air you can get into a gas engine, the more power it will make, but also the more likely it is to knock and ping (or the higher octane it will require). For that reason, cycles and other gas engines that are intended to run on pump gas (super) with a turbo or blower generally have lower compression pistons. Also, the strength of the engine’s bottom end may play a role, especially if longevity is an issue. Supercharged, high compression gas engines are used in drag racers, for example, but they run only on racing gas, and even with high $$$ forged rods and pistons, engine life expectancy is measured in quarter miles.

For diesels (someone correct me if I'm wrong) the more air you can move through the engine, the more fuel that can be injected without exceeding safe exhaust temps, at least to a point. Thus the more power that can be made. Diesels are not subject to octane ping/knock, which limits compression/boost that can be used in gas engines. High compression and boost are part of what makes these engines about 1/3 more efficient.

While it is true that high compression and boost in a diesel does put increased stress on rods/crank and pistons, good engines are designed accordingly. The pistons, rods and so on in the Cummins are VERY ROBUST, and designed to live a long life even with high compression and boost. The Cummins runs a compression ratio of 17.2:1 with significant turbo boost, where as the 360 gas V-8 runs 9.1:1 normally aspirated. Without slamming the gas engine, the Cummins also has a much longer expected useful life.

So anyway, "high compression" can be considered a relative term. I once had an AMX that ran 10.2 compression, and it would ping on super. The engine was designed for leaded gas, which resists pinging better. If it had the 17.2 ratio of our stock cummins, it wouldn't run at all...

Someone feel free to correct me if I got it wrong…

Jeff

 

You have to read that whole article to really understand what they're talking about. They're not saying diesels need a lower compression than gassers...just that for semi trucks they don't need the high compression the stock engines are given. That whole article is geared toward very high mileage vehicles such as semi trucks w/ 1,000,000 miles on them. The reason they like to decrease the compression ratio is to decrease wear and tear. They compensate for the decreased compression ratio by INCREASING the turbo's boost pressure by around 30 psi. They also modify the high pressure fuel pump to put out more volume of fuel to compensate for the larger fuel injectors.

Now, before you think about doing that to your CTD you have to realize that you're most likely not gonna own it when it has 500,000 miles on it!

 

Thanks.

After going to the article... It sounds like what he is saying is that to make a high powered diesel, lower (by diesel standards) compression pistons coupled with increased fueling and boost are the way to go, which makes sense to me.

"Turbocharged diesel engines must have low compression pistons in order to produce high horsepower" taken out of context, though, did not seem to make sense.

I guess I sould have went to the article before I replied. Sorry.

 

Jeff is right on, but I'll add my nickel:

The use of high and low compression in the article refers to "normal" for that engine.

"Normal" CR for a gas engine is typically 9:1.

"Normal CR for a diesel is 21:1 or higher.

What Bruce is saying that while a gas engine needs MORE compression for performance, a turbodiesel needs LESS compression.

Gassers need the high compression because they are mostly naturally aspirated. The higher compression in an NA engine enhances breathing efficiency and power output.

But in a TurboDiesel, LOWER compression is the way to go-- IF you raise the boost at the same time. There's a reason for this.

You have to remember that compression ratio and expansion ratio are the same. So if you have a 20:1 engine and 2000 psi of pressure at TDC, then you'll have 100psi at BDC, all else being equal.
But if that engine has a CR of 10:1, then the engine would be left with 200psi residual pressure at BDC.

So we can see that the higher the compression ratio, the faster the peak cylinder pressure tapers off on the way down. This harms your AVERAGE cylinder pressure. HP comes from AVERAGE cylinder pressure (BMEP- brake mean effective pressure).

So low compression ratio is ONLY good (in terms of hp) if you can get a bunch more air into the engine. It will also be easier on bearings and such, but not by a large amount.

It's useful to think of a turbodiesel as a two stage air compressor. It's more effective to have more work done inthe first stage of compression, and not as much in the second. It takes a lot of effort to squeeze 20psi of boost by 18 times in that engine. Better to squeeze 30psi only12 times.

This shifts compression work away from the cylinders and to the turbo, which is operating off wasted heat anyway-- much more efficient.

Justin

 

OK so I get it now. Its really confusing but i understand. So according to what was said up there.... If we lower our CR and raise our boost our motors will last longer?

 

Not really. What he's saying is that if you crank the fuel and boost way up and don't lower the compression it'll blow up the engine. Remember that he's going for million mile durability in a hard-working truck. We get away with what we do because most of us don't work the engine hard and we don't run 'em that far. If you're going to build an engine for hot-shotting, you definitely need to do as Bruce Mallinson says. If you're just going drag racing and playing around on the street, the stock pistons will be OK.

 

also that article is refering to eng with stupid pumps.......I have known this guy for years (over the phone) bought parts and aplied his logic ....works.period.

 

You mean "stupid pumps" as in, pumps that aren't computer controled?

 

yes........he was getting 600to 8000 hp out of 855s long b4 cummins thought it could be done (reliably).....in fact they used lots of his logic latter on.
The fuel propiter was a invention of one of his customers.
So when they are talkin-listen They have been there. They took the MVT that nobody liked and figured out how to make it really work( Mechanical variable timing ) (fuel)