Analysis of Engine Mounting

[RobbyObby]

Has anyone ever done any real analysis on the forces reacted at the engine mounts by the actual engine? I know it should just be a simple statics problem but im having trouble figuring out the forces caused by the engine torque. Is it as simple as taking crank torque and multiplying by the distance to the mounts?
The only forces I can see making a big contribution are the forces caused by the engine torque and the forces caused by the weight of the actual engine under lateral and longitudinal acceleration. Are there any that I am missing?

[RobbyObby]

Has anyone ever done any real analysis on the forces reacted at the engine mounts by the actual engine? I know it should just be a simple statics problem but im having trouble figuring out the forces caused by the engine torque. Is it as simple as taking crank torque and multiplying by the distance to the mounts?
The only forces I can see making a big contribution are the forces caused by the engine torque and the forces caused by the weight of the actual engine under lateral and longitudinal acceleration. Are there any that I am missing?

[HenningO]

You might want to consider how the force (not torque) in the chain (if you are using one) is reacted...

[JasperC]

Question is, what do you consider "simple statics". Since your mountings are (probably) going to be statically indeterminate, you need to get an idea of the stiffness of the mounts if you want to get an idea of the forces they take. This makes the design of the engine mounts really interesting because basically you could make as many design iterations you want (guesstimate stiffnesses, calculate forces, design, calculate stiffnesses, re-calculate forces, re-design, etc...).

As for your load cases, don't forget vibrations, forces due to shifting, and even impact strength unless you want the engine to shoot through your driver in the event of an accident.

As a bonus I'd like to share this little movie with you guys: youtube.com/watch?v=uIuQOi5PXu4

This is a shot of our rear engine & diff mount (single piece) filmed by Airbus's highspeed camera at FSUK 2010. Obviously single cylinder which explains the huge vibrations. Everything on the car is shaking; the scatter shield, the suspension and much to our surprise the rear bulkhead of the monocoque is also vibrating all over the place. The only thing that doesn't really seem to flex is the engine mount itself, so our conclusion from this movie was that perhaps the engine mount was a bit too stiff. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Best regards,

Jasper Coosemans
Chief Drivetrain 2009-2010
DUT Racing Team
Delft University of Technology

[RobbyObby]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by HenningO:
You might want to consider how the force (not torque) in the chain (if you are using one) is reacted... </div></BLOCKQUOTE>

Wouldnt the force from the chain just be reacted to the sprocket/diff and therefore into the diff bracket?

The reason I brought this up is because our team has historically used 6 total engine mounts (Honda F4i) but this year I would like to both simplify and lighten our mounting scheme and move to 4 mounts since it would making packaging much easier for our setup. Now i realize the only true extreme that really needs be considered is the impact load in an front end collision, all the other forces IMO are no where near large enough to come close to the yield strength of the brackets. (Correct me if Im wrong though). I just want to try and figure out a quantitative value for reaction forces to verify the design/location of the mounting brackets.

[HenningO]

Robby,
The chain force is as big on the diff sprocket as it is on the engine sprocket (but in opposite directions).

I've just seen so many cases of poor load paths between the diff brackets and the engine (aka. rear engine mounts). It leads to compliance, which in my experience does nothing but decrease the life of the chain and other driveline components.

Jasper,
Thanks for sharing that! Truly remarkable how much everything is moving!

[oz_olly]

Hey Jasper, that is a truly remarkable video. It's quite amazing to see just how much everything actually moves. That would be a great video to show half way through the engineering degree when the drop the bombshell that in real life all objects are flexible to some degree.

Robby, have you considered the engine restraint in a 20g frontal crash? We now design for that case as we don't want the engine to break free, crush the fuel tank and beat the driver in the back during a crash. As said above most engine installations are statically determinate but you can help youself out by designing some mounts to be deliberately compliant in a one direction so that it only takes forces in a plane. Our new set up has the front mount taking vertical and horizontal loads while the rear mount takes all the longitudinal. I guess it would be safer to have some redundancy in the mounts but it's not easily achievable with the concept we're running (it wouldn't be very light).

Cheers

[JasperC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RobbyObby:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by HenningO:
You might want to consider how the force (not torque) in the chain (if you are using one) is reacted... </div></BLOCKQUOTE>

Wouldnt the force from the chain just be reacted to the sprocket/diff and therefore into the diff bracket?

The reason I brought this up is because our team has historically used 6 total engine mounts (Honda F4i) but this year I would like to both simplify and lighten our mounting scheme and move to 4 mounts since it would making packaging much easier for our setup. Now i realize the only true extreme that really needs be considered is the impact load in an front end collision, all the other forces IMO are no where near large enough to come close to the yield strength of the brackets. (Correct me if Im wrong though). I just want to try and figure out a quantitative value for reaction forces to verify the design/location of the mounting brackets. </div></BLOCKQUOTE>

Probably frontal collision is indeed your most severe static load case, but for the other load cases (especially the vibrations), I think you will want to look into the fatigue life of the engine mounts.

Again, I'm kind of "single-minded" and I have no idea how big the vibrations on a 4-cyl are, but better to check up on the fatigue thing now rather than after you find your engine lying on the ground due to fatigue failure of the brackets, I'd say. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Cheers,
Jasper

DUT Racing Team 2008-2010
Delft University of Technology