Piston Design

[DevinN]

Hi All,
Current GM V8 engines are available with different piston tops, performance applications like the LSx engines are fitted with flat piston crowns. In contrast many 6.0L Truck engines come with a dished piston crown.

What determines the piston design that is best for each application?

 

[tbuelna]

compression ratio.

 

[jbthiel]

It also has to do with quench and swirl characteristics. The flat top has the advantage of no obstructions for the flame front. The dished design tries to concentrate the charge towards the center of the piston so the flame front doesn't have as far to go to burn the majority of the air/fuel charge. About the worst design in this regaurds is a high compression 426 hemi piston. There is a huge wedge on the piston which obstructs the flame front, but the improved breathing make up for this.

You also need to consider the taget rpm range, bore diameter, and piton weight. I have seen many references that a bore of approximately 4" is a good compromise.

 

[DevinN]

Tbuelna, The very short answer leads me to believe you’re thinking the only reason to put dished pistons in the 6.0L is to lower the compression ratio. Adding a central dish with everything else remaining the same would lower compression.

In addition squish to bore ratio is lowered with the squish band of the piston. This could have been avoided with a different piston design or with a larger combustion chamber. Instead the designers decided on the central bowl, my desire is to understand these reasons.

 

[DevinN]

Jbthiel,
I’m digesting your response. A unique combustion space is created when a dished piston is at TDC, the squish area becomes a small band and the chamber extends under some of the normal squish area of the cylinder head. With this mismatch the flame appears to have a greater distance to travel, why would this be desirable?

 

[patprimmer]

All the theory about quench, volume of chamber, distance from plug to most of the charge etc applies to all applications.

What changes from application to application is requirements for power density vs fuel cost.

Sporty applications will favour power density and commercial applications will favour low fuel cost and long engine life. This means lower compression than sports models.

They choose to vary pistons rather than heads, I presume for manufacturing cost and inventory control reasons.

A reason for the shape of the dish might be to eliminate the need for left and right pistons or left side vs right side inlet valve pistons.

Regards

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[jbthiel]

Assuming near equal compression ratio and combustion chamber shape the distance between the flat part of the head and the piston is greater with a flat top.

On a couple of other forums the general consensus was that you want a quech of about 0.04". I'm not sure what the whole rational is - but the indications are that a space this tight along with modern combustion chambers will result in an efficient burn and help prevent detonation (and combustion along the edges until the piston moves down some?)

 

[jbthiel]

I forgot the mention - Pat has a good point about keeping the heads and rods the same, just modify the piston crown. Easiest thing to do for parts commonality.

Are you looking at two engines in particular? The LS family has at least 4 different bore sizes and 3 different strokes.

 

[DevinN]

Patprimmer, can you explain this concept?

“ sporty applications will favour power density and commercial applications will favour low fuel cost and long engine life. This means lower compression than sports models”

I understand the idea of reduced compression relating to extended engine life. The part that puzzles me is low compression in relation to low fuel cost. I would think that increased compression would lead to higher engine efficiency and reduced fuel costs.

 

[DevinN]

jbthiel

“Are you looking at two engines in particular? The LS family has at least 4 different bore sizes and 3 different strokes”

jbthiel, I am comparing the 5.7L LS1 engine that was installed in the Camaro at the end of the production run to the 6.0L engines that were installed in Trucks during the same time frame. I’m trying to understand the rational used when determining the engine design differences.

I understand the differences in the application in regards to the vehicles but not how the requirements of these applications relate to the engine design differences.

 

[patprimmer]

DeviN

Hi octane fuels are more expensive generally by a greater margin that the increased thermal efficiency of the higher compression ratio they will support.

If this were not true, all manufacturers would increase compression and the lower octane fuel that was not cheaper would disappear from the market.

jbthiel

the flat part of the head and the piston is greater with a flat top.

How so. All serious engine builders I know, cut the deck of the block to give what they believe is the optimum clearance. It varies with engine size, cooling, rpm and materials of construction, but so close they just touch at maximum rpm with hot pistons and a cold block, plus 0.002 or 0.003" is what most aim for.

This is to expel as much charge from this area as possible without anything actually hitting in operation. This charge being expelled then tumbles across the combustion chamber, thus increasing the speed of flame spread, thereby allowing for later ignition but still reaching maximum cylinder pressure at the ideal time.

Regards

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[GregLocock]

DevinN and pat raise an interesting, real world, point.

Over the years there has been some pressure to reduce the octane levels of petrol. Australian car manufacturers succesfully demonstrated that this hurt fuel economy, consequently our octane levels for 'regular' fuel are increasing. Also, some modern engines are calibrated such that the extra efficiency from high octane fuels more than offsets the increase in price, eg WRX on premium gas. Of course another way of saying that is that their efficiency falls off with decreasing octane, which is not quite so good PR wise.







Cheers

Greg Locock

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[jbthiel]

Patprimmer
1. I prequalified that statement with "Assuming near equal compression ratio and combustion chamber shape." My point was along the production commonality line - if the heads and CR are the same the flat top will need to stay farther down the hole. I never said it was ideal. And usually if you change piston design you will be changing CR.

2. I agree with you on an all out performance engine. Off the top of my head I was thinking about 0.010", but when I searched a couple of different forums to confirm this the 0.04" value was being tossed around alot. That seemed too big to prevent ignition.

What do you think the minimum design clearance is for a production engine? I wouldn't think that they could hold less than 0.040" with all the tolerance stack-up. Maybe thats were the bigger numbers are coming from?

 

[patprimmer]

I have seen race engine builders control compression with deck clearance, but I never saw it on a production engine. Same engine with different compression always had different piston top shape or different combustion chamber shape.

0.040" static is not 0.040" running. Blocks expand, pistons rods and cranks expand. Rods stretch, piston skirts flex and pistons rock.

The optimum clearance depends on the size of the engine, the construction style and layout, materials used, accuracy of plant, and the engines cooling system.

A SBC with steel rods generally likes about 0.036" but an air cooled VW with forged pistons and 93 mm bore likes about 0.032"

Regards

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[DevinN]

I’m mostly interested in the relation ship of engine design to the intended application.

The performance application is more focused on higher RPM power with an aerodynamic body style and gearing to match; this ultimately reduces engine load.

In contrast the Truck application demands an engine that will perform with heavy loads at a lower RPM. Body styles are less aerodynamic and consideration must be taken for additional payload and towing capacity.

Surly these distinct differences weigh in the decision making regarding piston selection and final compression ratio.

 

[patprimmer]

I thought I already kind of said that on 30/11/06.

Sporty applications will favour power density and commercial applications will favour low fuel cost and long engine life. This means lower compression than sports models.

The truck application will need to do higher mileage at higher load, so it will need lower rpm and generally heavier components or more durable material, but this does not effect piston dome shape other than its effect on compression. This might be reflected in block casting details and materials, crank material, 4 bolt mains, piston alloy and skirt design, valve train components etc

Regards

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[jbthiel]

The two primary drivers of mass production piston design these days is cost and emissions, with less priority given to octane tolerance, fuel efficiency, air flow, etc.

Your two engines of interest use the same stroke and only differ in bore size by 2.5mm (~0.1"). One reasone may be that the LS1 is a gen3 engine where the 6.0L is a gen4.

 

[DevinN]

Pat, I'm trying to relate you comments to specifications and reported performance indicators. The higher compression 5.7 engine is superior in efficiency as shown with MPG findings. This may be related to vehicle weight and aerodynamics. I am researching the two engines here's what I have found so far. The cylinder heads are unique for each application.

6.0L/364 CI Truck engine (Gen. III)

71.0 cc combustion chamber
9:1 compression ratio dished piston (estimate CR, I have not found actual)
300 HP
360 FP Torque
4 speed automatic
Consumer Guide Observed MPG 13
Fuel 87 regular

5.7/350 CI LS1 Camaro engine (Gen. III)

67 cc combustion chamber
10:1 compression ratio flat top piston
310 HP
340 FP Torque
4 speed automatic
Consumer Guide Observed MPG 17.4
Fuel 91 premium

 

[DevinN]

jbthiel said:
"The two primary drivers of mass production piston design these days is cost and emissions, with less priority given to octane tolerance, fuel efficiency, air flow, etc."

I agree with this totally, this is one of the reasons I'm interested in learning more. Trucks are widely used in my area and appear to be lagging in fuel efficiency.

 

[patprimmer]

Am I missing something here.

As I am reading your recent comments you are trying to associate the higher fuel consumption of a truck vs a sports car to piston design.

In my opinion, the difference in fuel economy will be almost entirely due to the weight and aero differences of the vehicle, but with slight differences due to compression ratio and barely measurable differences due to other influences of piston design.

I will say it again. The dish is to reduce compression so that cheaper fuels can be used, thereby giving lower running costs. All other factors being equal, lower compression also gives lower power output but longer engine life or equal engine life under a heavier duty cycle.

To quote an old joke. "Mummy mummy, why am I going round in circles? Shut up kid or I will nail your other foot to the floor!".

I am starting to feel like some one here has one foot nailed to the floor.

Regards

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