"wierd" combustion chamber?

[berfinroy]

I recently saw a profile of the 189 combustion chamber...offset to one side with the spark plug located on the side of the block, cylinder head/block interface at an oblique angle.
Given that it has been known for a long time that a hemispherical combustion chamber is ideal, I can't understand how Mercedes' engineers came up with this peculiar design, which was not in fact very efficient. It took other exceptional measures...long ram tubes, fuel injection, free flow exhaust and high compression... to get respectible power out of this engine in the SL application (actually about 180hp, not the 215+ that was claimed).

Is there anybody that can shed any light on Mercedes' thinking regarding the design of the 189 cylinder head/combustion chamber?

P.S. I know from recorded comments by Rudolph Ulenhaut that Mercedes' engineers had to "make do" with existing 300 sedan components in creating the 300SL, which means that the 189 engine design was intended for a sedate sedan rather than a sports car, but the question remains, why was the chosen design considered good for THAT application?

[Idle]

There is a lot going on in this area of the engine at this time and they all work together.

The lack of a combustion chamber in the head was a result of wanting to make a cylinder head as cheaply and as quickly as possible. Mercedes had to get back in the game quickly after WW2 and they had to go with what they had.

The cylinder head uses a smaller diameter intake valve in order to increase the velocity of the fuel mixture into the combustion chamber which is really just the top part of the cylinder. This is why the spark plug is located on the side of the engine. There was no room for it in the head.

The exhaust valve is larger in order to push out the burnt gases quickly since there is no vacuum to draw the exhaust out like there is to draw the fuel in.

In other words: It is important to get the fuel mixture in quickly and burnt before it has a chance to break apart. If it is not mixed properly it will not combust properly.

But the exhaust gas just has to get out, so the valve is larger. This valve also sees more heat so it can deal with the increased temps since there is more of it do heat up.

This design, which was 'borrowed' from GM tech of the late 30's, also allows the engine to be adjusted to the different octane levels available in the country where the car is sold. Low octane fuel needs low compression and this was accomplished by removing the head, installing a thicker head gasket, and then putting the engine back together. Mercedes also built the engines at the factory with the cars' original destination in mind, but if you moved the car from one country to another, one with higher or lower octane gasoline as the norm, you could easily adjust the engines' octane needs with this head/head gasket design.

The long runners were to even out the intake pulses set into motion by the action of the intake valves. This is too complicated to go into here but it is a reason you see these long runners on a lot of cars these days. To make runners like this out of metal, like Mercedes did, is very expensive. To make them out of plastic is cheap so now a lot of companies are using this design feature to grind out a few more HP.

Racing engines, however, used a different design that the production engines but in racing the factory could control the octane levels. Mercedes, like everyone, uses racing to test concepts so from the successful designs used in racing come the cylinder head designs we have today in production cars.

And the very best design of all? Pistons that are oblong shaped with six valves per cylinder. And as soon as someone finds a way to make an engine like this for less than $250,000 each we will be seeing this in our production cars.

Until then we will have to make do with what we have.

[Frank Reiner]

A bit of fortune cookie wisdom for Calendar Changing Day:

"It is better to hold one's tongue and be thought a fool, than to open one's mouth and remove all doubt."

[Idle]

Originally Posted by Frank Reiner

A bit of fortune cookie wisdom for Calendar Changing Day:

"It is better to hold one's tongue and be thought a fool, than to open one's mouth and remove all doubt."

I take it you are unhappy with my explanation of this cylinder head situation.

So please take a crack at it yourself.

I have been dealing with older engines like this for perhaps fifty years or so and I paid attention to the older mechanics and engineers while I was learning. It is possible that one or even two of them might have been wrong about what they said.

But all of them, over many, many years , were wrong?

Another old saying: Those who don't know what they are doing should not get in the way of those that do.

[Frank Reiner]

Originally Posted by berfinroy

I recently saw a profile of the 189 combustion chamber...offset to one side with the spark plug located on the side of the block, cylinder head/block interface at an oblique angle.

The predecessor of the M189 is the M186, and it did have the spark plugs in the block, and the block was iron. The M189 has an aluminum block, however the spark plugs are in the cylinder head.

Originally Posted by Idle

There is a lot going on in this area of the engine at this time and they all work together.

The lack of a combustion chamber in the head was a result of wanting to make a cylinder head as cheaply and as quickly as possible. Mercedes had to get back in the game quickly after WW2 and they had to go with what they had.

The M186 ("The Big Six") was introduced in 1951, in that same year the M180 ("The Little Six") was also introduced; the M180 has the combustion chamber in the cylinder head, and the upper block surface is at 90 deg. to the bores. The ability of DB to introduce two new engines in the same year suggests that they were not constrained by a problem of having "to go with what they had".

The cylinder head uses a smaller diameter intake valve in order to increase the velocity of the fuel mixture into the combustion chamber which is really just the top part of the cylinder. This is why the spark plug is located on the side of the engine. There was no room for it in the head.

An examination of a head will reveal that the larger valve is the intake.
The M189 version has the spark plugs in the head; there is plenty of room.

The exhaust valve is larger in order to push out the burnt gases quickly since there is no vacuum to draw the exhaust out like there is to draw the fuel in.

The smaller of the two valves is the exhaust valve; the pressure differential between cylinder and exhaust pipe during the exhaust phase is much greater than atmospheric, hence, exhaust gases can be expelled thru a smaller valve.

In other words: It is important to get the fuel mixture in quickly and burnt before it has a chance to break apart. If it is not mixed properly it will not combust properly.

But the exhaust gas just has to get out, so the valve is larger. (see above) This valve also sees more heat so it can deal with the increased temps since there is more of it do heat up.

This design, which was 'borrowed' from GM tech of the late 30's, also allows the engine to be adjusted to the different octane levels available in the country where the car is sold. Low octane fuel needs low compression and this was accomplished by removing the head, installing a thicker head gasket, and then putting the engine back together. Mercedes also built the engines at the factory with the cars' original destination in mind, but if you moved the car from one country to another, one with higher or lower octane gasoline as the norm, you could easily adjust the engines' octane needs with this head/head gasket design.

The "borrowed" design appeared in the M180 engine, not the M189.

The long runners were to even out the intake pulses set into motion by the action of the intake valves. This is too complicated to go into here but it is a reason you see these long runners on a lot of cars these days. To make runners like this out of metal, like Mercedes did, is very expensive. To make them out of plastic is cheap so now a lot of companies are using this design feature to grind out a few more HP.

The runner length is selected, not to "even out the intake pulses", but to provide resonance at a chosen RPM that results in increased intake charging, with a corresponding increase in torque.

Racing engines, however, used a different design that the production engines but in racing the factory could control the octane levels. Mercedes, like everyone, uses racing to test concepts so from the successful designs used in racing come the cylinder head designs we have today in production cars.

And the very best design of all? Pistons that are oblong shaped with six valves per cylinder. And as soon as someone finds a way to make an engine like this for less than $250,000 each we will be seeing this in our production cars.

Until then we will have to make do with what we have.

[berfinroy]

Mea culpa. I didn't realize that the first single carburetored version of the 300 series engine was designated M186, followed by M187 (twin carburetors) and 188 (triple carburetors), while the later fuel injected version was designated M189. The carburetored versions did have the spark plug in the block, not the cylinder head, whereas the M189 placed the fuel injectors where the spark plugs had been located and moved the plugs to the cylinder head.

I don't believe the cylinder block of the M189 engine was made of aluminum until 1960-61 and thereafter for all applications until the engine was discontinued in 1967.

[Frank Reiner]

Originally Posted by berfinroy

Mea culpa. I didn't realize that the first single carburetored version of the 300 series engine was designated M186, followed by M187 (twin carburetors) and 188 (triple carburetors), while the later fuel injected version was designated M189. The carburetored versions did have the spark plug in the block, not the cylinder head, whereas the M189 placed the fuel injectors where the spark plugs had been located and moved the plugs to the cylinder head.

I don't believe the cylinder block of the M189 engine was made of aluminum until 1960-61 and thereafter for all applications until the engine was discontinued in 1967.

Only the M198 (300SL application, iron block) placed the injection nozzles in the block, in the former spark plug location. The M189 (aluminum block) placed the injection nozzles in the intake runners adjacent to the head/runner joint.

[berfinroy]

Originally Posted by Frank Reiner

Only the M198 (300SL application, iron block) placed the injection nozzles in the block, in the former spark plug location. The M189 (aluminum block) placed the injection nozzles in the intake runners adjacent to the head/runner joint.

Thank you, Frank for this additional clarification. I presume that your deferentiation would also be correct for ALL iron vs aluminum block 189 fuel injected applications, such as the 300d fuel injected iron blocks (injector in block) and later series aluminum block (injector in intake runner) 300SL, 300 fin tails and 109s?

[Frank Reiner]

Originally Posted by berfinroy

Thank you, Frank for this additional clarification. I presume that your deferentiation would also be correct for ALL iron vs aluminum block 189 fuel injected applications, such as the 300d fuel injected iron blocks (injector in block) and later series aluminum block (injector in intake runner) 300SL, 300 fin tails and 109s?

Mister Roy:

Herewith an excerpt from Wikipedia on the M186 series; it indicates the existence of the M199 version (iron block, direct injection). It is a version of which I was unaware.
This Wikipedia article, then, indicates that the 300d used the indirect-injected engine (M189), and the 300Sc the direct-injected engine (M199).
M189 > aluminum block. M198/199 > iron block.

M189
The first M189 appeared in the 300d pillarless limousine of 1957, sporting Bosch indirect fuel injection and producing 180 PS (130 kW; 180 hp) at 5500 rpm.^[3] The engine then appeared in the Mercedes-Benz W112 300SE of 1961 and its long wheelbase variant two years later. From 1964, power output was 170 hp (130 kW). The final incarnation of the M189 was in the W108 300SEb and W109 300SEL of 1965-1967, producing 170 hp (130 kW).

M198
The iconic gullwing 300SL and its roadster counterpart featured a Bosch mechanical direct injection version of the engine, producing 215 hp (160 kW) from 1954 to 1963.

M199
A slightly detuned direct-injection engine was used in the W188 300Sc coupe of 1955 to 1958 producing 175 hp (130 kW)

[ejboyd5]

Relative to this discussion on M-B engines, some points need to be clarified:
1) The M 187 engine was not only equipped with three side draft Solex carburetors but more importantly had a Roots supercharger. Power transmission problems through the existing rear axle arrangement limited use of this application in the racing arena.
2) The M 186 engine was the precursor of the engine installed in the production 300 SL. This engine was notable for having the parting line between the block and the head set at 20 degrees to the horizontal thereby allowing the valve faces to be parallel to the head surface with the combustion chamber existing in the wedge shaped area in the block created by the angular difference between the top of the piston and the parting line.
3) The intake valves were indeed larger than the exhaust valves to maximize flow by capitalizing on the different pressures obtaining during the intake and exhaust strokes. Interestingly enough the valve stems were larger on the smaller diameter exhaust valves to allow room for their hollow sodium salt filled cores intended to improve valve cooling.
4) The 17" runners on the intake manifold were not there for pulse dampening, but rather were created to allow maximum flow at 5500 rpm, a speed that nested comfortably between the maximum torque and horsepower revolutions.
5) The light metal M 198 block first appeared in Chassis Nr. 109.042-10-003049.
6) All M 198 engines in the 300 SL, whether iron block or light metal block, had the injector nozzles installed in the block and the spark plugs installed in the head. The main reason for this configuration was the difficulty encountered in changing spark plugs mounted in the block when the engine itself was inclined 50 degrees to the horizontal in the 300 SL as opposed to the previous upright mounting in passenger cars.

[berfinroy]

ejboyd5:
What racing application of the 187 engine used a Roots supercharger?

Your explaination of the spark plug/fuel injector arrangement in the 300SL makes sense. I assume from this that having the injector in the intake runners and the plug in the block was the preferred arrangement.

No one has quite answered my original question as to the thinking behind this peculiar and never repeated combustion chamber in block design.

[Frank Reiner]

Originally Posted by berfinroy

ejboyd5:
What racing application of the 187 engine used a Roots supercharger?

Your explaination of the spark plug/fuel injector arrangement in the 300SL makes sense. I assume from this that having the injector in the intake runners and the plug in the block was the preferred arrangement.

No one has quite answered my original question as to the thinking behind this peculiar and never repeated combustion chamber in block design.

Unable to speculate on the thinking, but the design was repeated in the 348/409 cu. in. W-series engines from Chevrolet 1958-63.
From Wikipedia:

Unlike many of its contemporaries, the combustion chamber of the W-series engine was in the upper part of the cylinder, not the head, the head having only tiny recesses for the valves. This arrangement was achieved by combining a cylinder head deck that was not perpendicular to the bore with a crowned piston, which was a novel concept in American production engines of the day. As the piston approached top dead center, the angle of the crown combined with that of the head deck to form a wedge-shaped combustion chamber with a pronounced quench area. The spark plugs were inserted vertically into the quench area, which helped to produce a rapidly moving flame front for more complete combustion.

[berfinroy]

I see your point, Frank, although the Mercedes combustion chamber is markedly offset from the cylinder bore, whereas the Chevrolet combustion chamber is in the cylinder bore itself.
Mercedes' design is almost certainly more expensive to produce, and as far as my limited understanding of combustion chamber design goes, pointlessly so.

Comment by somebody with advanced understanding of the esoteric niceties of air/fuel mixture "swirl", "propagation of flame front" and the like would be interesting

[Frank Reiner]

Originally Posted by berfinroy

Mercedes' design is almost certainly more expensive to produce, and as far as my limited understanding of combustion chamber design goes, pointlessly so.

AMEN!

My recollection of events and performance of those (Chevy) engines at the time, suggested to me that the best use of that chamber design was in truck applications, where engines run at relatively low RPM, but with high manifold pressure. That chamber seemed quite resistant to detonation, a desirable property for a truck engine.

Never the less, the high performance versions did quite well, on a par with 406 Fords, and 413 Mopars.

[ejboyd5]

Questions by berfinroy:

What racing application of the 187 engine used a Roots supercharger?

Although entered in an 8 litre class for the August, 1952 Nurbergring race, the two M 187 powered cars (a coupe and a special short wheelbase roadster) never actually competed having been withdrawn following reliability concerns caused by the supercharged engine's voracious appetite for head gaskets.

Your explaination [sic] of the spark plug/fuel injector arrangement in the 300SL makes sense. I assume from this that having the injector in the intake runners and the plug in the block was the preferred arrangement.

No 300 SL engine (M 198) ever had the injection nozzles in the intake runners. All engines, both iron and light metal blocks, had the nozzles in the block and the spark plugs in the head.

No one has quite answered my original question as to the thinking behind this peculiar and never repeated combustion chamber in block design.

Actually, the design incorporating the 20 degree slant of the parting line between the block and the head was the result of considerable thought and planning allowing for the placement of relatively large valves having their faces parallel to the lower edge of the head combined with what was considered a well shaped combustion chamber created by the wedge shape of the block/piston arrangement and the sculptured shape of the piston face. This combustion chamber really came into its own with the addition of the fuel injection nozzles to the side of the block allowing the fuel spray to travel from the hot side of the chamber (exhaust valve side) to the cool (intake valve side). Some may argue with the wisdom of this thinking from the vantage point of 2016, but it is very difficult to argue with the successes achieved in the 1950s with this arrangement.