Engine Variations

Hi guys,

I'm no engineer, but can someone kindly explain whats the difference between a 72 degree, 90 degree, and straight line engine?

What are the pros and cons?

Thanks in advance!

Brian

 

It's basically the angle of the "v" shape of a v4,v6,v8 etc engine. An "inline" engine has all the cylinders in a row.

 

Inline engines are really only practical up to 6 cylinders -- otherwise, the crankshaft and camshaft(s) get too long and noodly (and costly to make). One simplifying advantage for the inline configuration is that you can have just one intake mainfold on one side, and one exhaust manifold on the other.

V engines give a much more compact (lengthwise) package. If the bank angle is small (like 90 deg or less), you can have one shared (intake) manifold, but you need two of the other (exhaust) manifolds. If the V angle is large (like 120 deg or more), then you (usually) need two manifolds of each type, and more complicated plumbing/throttle control (small carb VW flat 4 cyl are an exception to this, but their intake manifold is sort of ridiculous ).

For large front-engine/rear-wheel drive cars, it was/is sort of a toss-up whether to use a I6 or a V6 engine (and most were either I6 or V8). For smaller front-engine/front-wheel drive cars, the available space (typically) limits you to I4 or V6 packages.

 

The inline 8 was one of the best engine configurations ever. It was very common in high end (read very large) pre-war cars.

The most inherently balanced design possible. Very smooth.

Inline 8 (and 6 as well) went out of favor because they are too long to fit into todays shorter cars.

 

Size was only one reason. Straight eights just cannot put out the power of a V8 and when it comes to power per LB they really lose out. One big reason, as Steve pointed out was the crank structure. The only straight 8 to achieve any modern success was the Mercedes W196 and W196S engines of the mid 50's. They were really for all intents, 2 4cyl cranks, end to end and by modern standards their power and RPM limitations are not very impressive.

Wonderful motor for the 30's and 40's. The 50's left them behind and correctly so.

 

John- 90 degree V8s with 90 degree crankshafts have no primary or secondary imbalances and are, as stated, compact in size. Too much compromise for a straight eight crankshaft between stiffness, weight and number of main bearings. Those crankshafts were really long.

I still remember my parents' 1948 Roadmaster convertible. Fine then, but soon replaced by Buick with a V8.

 

Don't forget the inline 5 .. my old Audi had that. When I'd buy spark plugs they'd ask .. do you already have one plug or are you buying a spare one? 4 and 6 seem to be what everyone has in mind for an inline engine.

 

I said the MAXIMUM number of cylinders for a practical inline configuration is 6 -- you can have an I5 -- if you are a crazy person!

 

Thank you so much guys. It's actually regarding a job interview. Will post more questions if i have any

 

*Many* years back I was in the scrutineering line at the first race of the season. Ahead of us was a Super-Vee car and it obviously had not one carb and huge inlet tracts but rather two mounted "correctly"....

"Err, we're pretty sure the rules say you can use any carb, with any # of chokes, but only one of 'em."
"Yep, that's true. But, it is only one carb..... A triple choke Weber cut in half with the middle choke deleted."
"Oh!"

Much head scratching in the scrutineering bay before they passed it. The following season "carburation is free"......

Cheers,
Ian

 

Ingenious!

 

Brian, the others here have already given perfect explanations, just one addition: The power (torque) output of an engine is not continous, but oscillates periodically. During the intake, compression and output strokes, it needs a certain amount of power to move or compress the gasses in the cylinder, only during the expansion stroke, when the fuel/air mixture burns, you have an actual power output. Additionally, you have to move the masses of the pistons etc, which nets zero, but oscillates too.

Cleverly arranging the firing order of the cylinders and their layout helps smoothing the power output. In addition to packaging issues (length/height etc. of the engine), this is the reason why we see different crank angle configurations etc.

 

Some people think the Testarossa engine is a V-12 but with the V angle at 180 degrees...

(I just could not resist it)

 

You mean like Ferrari themselves?

 

The W-196 Mercedes Benz straight eight racing engine of the fifties. (if my memory is correct) I believe the I8 had the crankshaft power take off in the middle to eliminate flex. Geared to a take off shaft.



Ago

 

Yes, but not like Porsche...

 

David- Affirmative, like Brian said, essentially two I4s in a common crankcase. Ferrari V8s with their 180 deg cranks are essentially two I4s on the same crankshaft. Most V8s have 90 degree cranks and are four V-twins on the same crank.

 

And, while I'll try to stay out of the flamewar about whether it's a 180-degree V-12 or a flat 12... There are in fact two flavors of flat engines:
- Two conrods per crankpin, as in a standard V engine. When the piston on one side is at TDC, the other is at BDC. The BB and TR 12s are of this type.
- One conrod per crankpin, so in each pair of opposing pistons, both are at TDC and both are at BDC at the same time. Its hard to imagine this design being applied to a 12-cylinder engine, but it it's a sweet and well-balanced setup for two, four, or eight cylinders.

 

A common term for an engine that shares the same crank pin is a V, no matter what angle of the V. I believe a true boxer is an engine with separate crank pins for each con rod.
The Porsche 917 12 cyl. racing engine had two configurations, one had two 911 cranks coupled together and had separate crank pins for each con rod. and another had a long crank that the con rods shared the same crank pin like the 12 cylinder TR engine. Porsche also had a 16 cyl. engine on the dyno, if the 917 12 cylinders didn't win.



Ago

 

This gave me a flashback to about 1979. My cousin and I were in the Porsche Museum in Stuttgart. No one was around so we decided we would each climb into the Porsche 917 they had on display and try it on for size. It was a cool view from the inside.

 

I was not aware of that...the only 917 engine drawings I have seen are paired crank throws - unlike the 911 which was a true boxer. It did have the same bore/stroke of the current 911 engine, but the different crankshafts or 16 cylinder type is not mentioned in the history of the 917 development that I have.

It should be noted that both of these variations would require a completely new engine - not just the crankshaft. The actual racing engine had four main bearings on each of the two 6 cylinder halves.

 

I found this picture (from 917 flat 12 turbo engine) and this drawing (from early 917 flat 12 naturally aspirated engine).

I think the displacement was 5.4 because it was basically 2 x 2.7 flat 6 engines from 911.
Initially the displacement was 4.5 because at that point the displacement of 911 was 2.2...

How cool is this... Check out the gears (no belts or chains) that operate the camshafts.

No wonder the 917 was so successful !!!
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Yes, that is the drawing and cutaway that I have seen. Note that a longer engine (like a 16 or a boxer 12) would probably have also required a completely different frame for the car because of the length increase.

 

Most of the early 917s had several sub-frames as they were made of aluminium and tended to crack after a few races..... The P team used compressed air to determine if the sub-frame had cracked.

 

Right - they had the tube frames carefully constructed to hold compressed air and a filler valve on each section. It was a pre-race checklist item to measure the pressure.

I cannot imagine what kind of courage it must have taken to run these things at top speed down the Mulsanne straight...