Petrol Engine Question

Hi all,

I'm currently sketching out some designs for a petrol-powered vertical spinner (Triumph, themed after 1950s British bikes), and I was wondering what a suitable weapon motor would be. I definitely want to use a petrol or diesel motor and I'm curious as to what's out there. My initial thoughts were to use some kind of tuned-up dirtbike engine in the 75cc range, but it might be a bit weedy compared to the LEMs and E-Teks of the world. I'd really appreciate the input of more experienced roboteers on this - for instance, I know that the motor on Icewave of Battlebots fame apparently has an output of 11 and a bit kW, but not where I'd find something with a similar power output.

Thank you all in advance for your comments. =]

Love this! I've wanted to see a petrol powered vertical spinner for ages. Most weapon motors are in the 4-12hp range, so anything 4hp or more would probably be suitable. Something like a hand held stone cutter engine is a popular option for this kinda thing.

Looking at displacement VS power output, yo will need around an 80 to 100cc 2 stroke engine to get the 4KW entry level power you need. Perhaps a stripped down 125cc bike engine might work

I was all for a Go Kart engine before the electronics specialist on the team ruled it out because LiPo fire + Petrol = death.

That being said, they're pretty good for power density, and if you know your way around engines, 50Hp isn't out of the question. There are pretty much two bulletproof engines out there - the Yamaha KT100 and the Rotax 125.

I'll start with the Rotax 125 - mainly because I've tuned/built a few. It's a lovely engine, mechanically speaking. Terrible to work on, fiddly as hell, but once its working, they've been those engines that still run to peak power after a crash. With a little work on the cylinder and piston assembly, you can get it to 11.5:1 compression, and if you buy a larger fuel delivery system (I was using .065'' jets the last time I worked on one), grind out the heads (or buy larger ones) it will be very inefficient (and very, very loud!) but will last the 3 minutes of the fight, with around 50Hp at peak power, which would probably be in the 12000ish RPM range. Torque is OK - stock I think it's 20Nm at peak.

The Yamaha KT100 is both lighter and slightly cheaper than the Rotax. It's also got a lot less power - around 15Hp at stock, but that can rise to around 35Hp if you bolt on the right speed parts. It's also a little more torquey than the Rotax, and it's been used in literally everything - Go Karts, Ultralights, trail bikes, kids hovercrafts - it's been around since the 70's, IIRC.

Allocate 30ish KG for the engine/fuel and you should be OK.


Three things you're going to have to watch out for if you do use a Go Kart engine

1 - Heat. The exhausts on these get hot. 250C isn't out of the question. They harder you run, the more likely you are to get some extreme head off of the header. The exhaust will need to be positioned well away from your batteries and your removable link.

2 - Inverted running. These engines pretty much exclusively use carburettors, so you'll either need to upgrade to fuel injection (which I honestly don't know if they do) or use a float reserve carburettor - same as they did on the Spitfire, but you'll have the same problem as the Spitfire, in that if you're upside down for too long or pull some negative Gs (read: getting flipped) your engine will starve for fuel.

3 - Torque delivery. You'll need a hell of a clutch to deliver the full torque of the engine to a large spinning bar or disc. Plus, the engine doesn't deliver full torque at idle - the peak torque on both of these engines are around the 8-9k mark.


A note on modifying these engines - especially the compression side of things. More compression makes it harder to start, but delivers better horsepower. The drag car that I do engine work for runs at 16.4:1, and requires 3 truck batteries to start it, and even then you only get 2 chances. Most street engines run at between 8:1 and 10:1

Hi everyone! Thanks so much for the ideas, tips, and support. I really appreciate it.

I've elected to follow emisnug's recommendations and go for a Rotax 125 as Triumph's main power plant. Additionally, I'm also going for sealed lead-acid batteries, as (should something go a bit wrong) this will lead to less, y'know... fire.

With regards to the rest of the robot's construction, I've been thinking about this and I reckon I've come up with something interesting. Given that between the weapon and the fully kitted-out engine 55-60kg is down the drain from the start, I've elected to use a rollcage design to save weight... but not just any rollcage. According to the SSAB site, you can get 4mm HARDOX in tubular form with an internal diameter of ~88mm. Quite by chance, I also know that you can get UHMWPE in circular rods in a variety of diameters, including between 60 and 80mm. My plan for the armour is therefore as follows:-

1: Drill/cut bolt-holes in the HARDOX and UHMWPE. I hear that the latter is easy to machine, and that the former... I hear that the latter is easy to machine. =]

2: Wrap a few layers of 2-4mm shock absorbent gel sheeting (such as this stuff) around the UHMWPE rod and fit it together. I should point out that if the gel is insufficient for my purposes, I can always just go for thicker rubber shock-absorbent sheeting and use thinner plastic rod.

3: Cut holes in the shock absorbent sheeting over the ones drilled into the plastic rod.

4: Shove the plastic and gel/rubber down the HARDOX tube.

5: Bolt it together with M10s or M12s.

6: Weld into place as necessary.

The thing about UHMWPE and similar plastics is that they're really light, but they're not much fun to try and weld. Our good friend HARDOX, on the other hand, is much less tricky to weld (so I have been told), and a tube with walls of thickness 4mm essentially gives you 8mm armour. Okay, so it won't be 8mm wear-resistant steel and between 50 and 80mm of techy space plastic everywhere, but it should stand up reasonably well to hits from big, nasty spinners. Will I be able to get it to work? Maybe. I am not a very technical person. I once thought giving a robot a gambeson made of quilted kevlar with chainmail sewn into it would be a practical armour solution.

But I reckon I'm onto something here, and I'd appreciate more guidance from experienced members of the community.

Thank you all again for your support in this slightly mad endeavour. I hope this works. =]

You need a STIHL chainsaw engine. Clutch and chain drive as standard...

Any reason you are looking at UHMWPE over HDPE? As HDPE has basically the same strength and density values but is a tonne cheaper.

What does the gell-wrapped UMHW achieve structurally? If you want more strength and rigidity, just increase the tube diameter and it will probably be lighter than the plastic core.

Quote:

---

Originally Posted by overkill

What does the gell-wrapped UMHW achieve structurally? If you want more strength and rigidity, just increase the tube diameter and it will probably be lighter than the plastic core.

---

looking at the website it talk about deadening vibrations but in context of hard drives and light components. Not sure warping the UHMWPE would achieve

You'd be better to dampen vibration between the chassis and your components or build the vibration dampening into the chassis.

I'd also get in touch with a few steel stock holders as I suspect hardox tube won't be that readily available and may scupper your plans early on.

I'm quite interested to see how this concept pans out having built
a petrol engined spinner myself for the series 6 of Robot Wars (auditions)
My spinner was a vertical but was more like a steel proppellor blade
and spun in front of the robot as such, it was running off a briggs
n stratton lawnmower engine which drove a heavy hub disk brake
and 50mm thick steel proppellor and a grinder addition.
I'm interested in how you intend to keep it running upside down
ie when its flipped, how you will cope with the fuel limitations 200ml
I think-you will need a Rc controlled electric start because its bound
to stall, It will have to be braked and if its not automatic you will need
a way to operate the clutch.
Interesting prospect but not an easy build.

Having looked at Hardox tubing for Deadlock the minimum order was like a kilometer or two.

Always good to have spares Rob :)

200ml of fuel= 1.78Kw of energy. Efficiency of a small 2 stroke petrol engine, 20%= 450W used for the disk.

Small customized 5Hp petrol engine + fueltank+ electric starter = +/-10 kg.

5Kw electric motor+ 12S 10 000mAh batteries*+ ESC= 7kg.

*Almost 450W, comparable to the usable power of the fuel.

And the electric motor happily runs upside down.

And is a lot smaller.
And is allowed on most events (where spinners are allowed)

Yeah but more importantly electric motors aren't nearly as cool..

Do a "Tesla Brabus" and install a soundsystem to make the noise.