Engine selection based on chassis dynamics

[540ZCar]

I'm stuck in a hard place. Both the 13B Renesis (Mazda rotary) and EJ25 (Subaru boxer) are accredited with lightweight and low center of gravity. The weight differences (based on what I've found) are marginal from each other, both roughly 260LBs but I think the long block of the EJ would have a lower center of gravity but since the easier packaging of the 13B you could mount the turbo low down and make custom brackets to hold the accessories lower as well would would probably make it have the same CG of the the EJ (this is all theory in my head).

Then I started thinking about moment of inertia, since the EJ spreads further out towards the sides then it would help the car resist the temptation to roll while the compact 13B in the center of the chassis would have a lower MOI so it wouldn't help resist roll at all. This might be a deciding factor, but it could also be a marginal effect not worth considering. Based on a 2100LB car with 50/50 L/R and 40/60 F/R, what would you think about this MOI theory?

Can you think of any other considerations besides torque/horsepower differences?

 

[GregLocock]

weight is the dominant consideration

then cgz

then cgx

roll MoI ranks about equal with color IM()O

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[540ZCar]

So I'm thinking too much into it?

 

[BrianPetersen]

Definitely. Just pick the engine that fits best and is most favourable for weight distribution (usually the lightest one that's consistent with power output).

 

[izzmus]

The Subaru engines as practiced are not particularly low. The main mass of the engine sits rather high, so that the exhaust and oil pan can have sufficient ground clearance. They do win points over other all wheel drive designs by having a relatively short, lightweight engine in front of the front axle, while putting the bulk of the transaxle behind the front axle.

The rotary engines are not particularly easy to package, either, since they are practically as wide as a boxer engine. Inline engines, and to a lesser extent V engines, can carry their manifolding up high to clear framerails and steering mechanism. Rotaries do not have that option.

 

[patprimmer]

Clearance of the sump is set by height of the crank no matter what the rest of the configuration, so with a boxer layout the height of the cylinder centreline is the same as the crank centreline or considerably lower than in an upright layout. The heads may be asymmetrical to the cylinder centreline to help with exhaust clearance, but still no where near as high as in an upright layout.

Regards
Pat
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[540ZCar]

If you don't mind, I'm just going to sit back and observe the the discussions that arise. I don't really have any questions at the moment but I love reading stuff like this.

 

[NormPeterson]

At some point, flywheel/starter ring gear clearance can become a (hard) limiting factor. This does assume that production-based components are being used.


Norm

 

[NormPeterson]

With respect to MOI in roll, if one is going to bother to consider its effect from "at rest" on corner entry, maybe its "in motion" effect during slalom-type maneuvers should also be looked at.

It still seems like noise.


Norm

 

[BrianPetersen]

A turn-in situation is the only one I can think of where the moment of inertia could be relevant. In that case, if you picture the moment the turn-in starts, the front wheels are steering at an angle and the rear wheels are straight, so the car is (instantaneously, and excluding the effects of tire slippage) pivoting around a point that's in line with the rear wheels. In that case, the radius from the pivot point (in line with the rear axle) to the parts of the engine is all going to be about the same, isn't it? In that case, the weight of the engine matters, the moment of inertial scarcely matters.

In a motorcycle, that initiates a turn by first pivoting in large-magnitude roll along a longitudinal axis before actually starting to turn, it's a different story and a narrow, compact engine with its mass concentrated closely around the center of gravity (think Ducati across-the-frame V-twin) ought to have some advantage over an engine with its mass sticking way out past the bike (think BMW flat-twin or Honda Gold Wing flat-six with cylinder heads projecting way out into the breeze).

In a car ... it just isn't going to matter much.

 

[BrianPetersen]

One other thing regarding the Subaru engines. True, their mass is concentrated around crankshaft height. But, the nature of the drivetrain layout forces the crankshaft centerline to be above the axis of the front differential and it forces the whole engine and clutch/flywheel to be ahead of the differential, which is sitting underneath and behind the clutch and flywheel. Other drivetrain layouts (transverse with differential behind the engine, longitudinal rear drive with no front differential to get around) aren't subject to these restriction or at least, not as badly. Some longitudional/front drivetrain layouts have the diff sitting beside the oil sump with one shaft going through the sump, but with a flat-four layout, ya can't do that because the cylinders are right there. Crank centerline still has to be above the final drive with that setup, but probably by not as much and at least it doesn't force the whole engine to be ahead of the front wheel axis.

For a different example, on a VW Golf-chassis vehicle, the crank centerline isn't much different in height from the diff (the oil pan is pretty low to the ground, which has its own set of disadvantages) and I know the engine is leaning significantly backward, which gets the weight of the cylinder head back towards the firewall and not as far ahead of the front wheel axis.

 

[GregLocock]

Let's see

MoI car ~1000*1^2 =1000

MoI engine ~150*.4^2=24

MoI engine that is 25% more compact 150*.3^2=13.5

difference in response frequency of car =1-sqrt(oldMoI/NewMoi)= 1/2%

So, if the weight transfer took 100 ms with the fat engine, and 99.5 ms with the thin one, who (a) cares and (b) even knows which is better?



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[540ZCar]

Actually mathed it out? That's pretty cool, I don't have any formulas that deal with MOI. The closest I have to that are ones that tell how much weight is transferred from left to right or front to back based on weight, CG, wheelbase/track, and Gs.

 

[izzmus]

Audis also have a forward mounted engine, and their crank centerlines are positioned a lot lower than Subaru engines.

Subaru places their steering beneath the drivetrain, so by necessity the drivetrain has to sit rather high. This may be why they don't seem to concerned about having a shallow sump or close-fitting exhaust - the engine has to sit that high anyway, so why not take advantage of it? Audis, on the other hand, have the steering mounted high, and the drivetrain is slung beneath it.

Interesting that the two-cylinder-long engine in my ex-Subarus held 4.5 quarts of oil, while the five-cylinder-long engine in my Audi product holds one quart less.