electrically forced induction 70cc 2 stroke

[kaelem]

seen a few threads speaking about how electrically supercharging an engine isnt viable because of the amount of power needed to compress more air than the engine usually draws in, however would this be the case on a small engine? thinking of making a way to compress the air fuel mixture in my 70cc 2t after the carburettor, would this actually be a possibility or is it just unviable for similar reasons to a larger engine?

 

[BridgeSmith]

Not sure what you are trying to ask. Superchargers are used specifically because they increase the output of the engine. The engine uses more fuel relative to the mechanical energy output in pretty much any engine. However, they do allow smaller, more efficient engines to output greater HP when called for. Whether that increases fuel mileage overall, vs. using a larger engine, is a function of how often it is engaged.

 

[3DDave]

A direct drive supercharger has the lowest losses, but also the least adaptability. Most often they are used at peak power conditions so adaptability isn't needed. Adding more electrical losses isn't helpful.

There is little lag in direct drive superchargers, though I recall some electro/exhaust driven system to decrease lag on a turbocharger.

 

[kaelem]

I should have been more specific, I often describe things how I understand them just to find they make little sense to anyone else . So the idea is to have an electric motor driven air compressor in between my carb and intake manifold so I’d be pressurising both the air and fuel going into the crank, I’m under the assumption that the electrical draw may well be worth it on such a small engine platform as it doesn’t draw massive amounts of air, a pulley driven or exhaust gas driven compressor isn’t really an option as I have such limited space anywhere other than directly around the intake manifold. I hope this clears up what I’m asking a bit thank you for your replies

 

[kaelem]

And my other thought on this is that perhaps running it off of the ignition pickup which is pulsed it would only be compressing the air fuel mixture when the piston is about to come on a down stroke so as not to over pressurise the crank and blow the seals

 

[GregLocock]

So, what power is your 2 stroke at what rpm?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BridgeSmith]

You did mean to say "cylinder" when you wrote "crank", right? If so, what you're describing sounds like a fairly standard supercharger. As far as I know, regardless of the size of the engine, you will not improve the overall efficiency (output energy / fuel input ratio) of the engine by adding a supercharger. It is not the purpose or the typical result. The purpose of adding any such device (supercharger, turbocharger, or blower) is to increase the available peak horsepower output of an internal combustion engine.

If I understand your question correctly, you're asking if a supercharged engine is more efficient than a non supercharged engine of the same size. The answer to that is no. Regardless of the size of the engine, the energy balance works out pretty much the same way.

The gain in energy efficiency is in being able to downsize the engine, while maintaining the peak horsepower available. I'll use the Suzuki 3 cylinder, 993cc (Geo Metro 1 liter) engine as an example, since it's more familiar to me, and I know the correct HP ratings. The peak output for the version with a carburetor is 52 HP, and for the fuel injected version, it's 55 HP, both naturally aspirated. The turbo version of that same engine has a peak HP output of 105 HP. The EPA estimated highway mileage for the carbureted version was 53 mpg; the FI version 56 mpg, and 48 mpg for the turbo version. However, to get an available peak output of 105 HP from a naturally aspirated engine, you'd typically have to bump up to something around a 1.8 liter displacement, and even in a car as small as a Metro, those rarely got better than 42mpg. Now, if you drove the Metro with the turbocharged engine conservatively, and the turbo never engaged, it would actually get very close to the same mileage as the non-turbo version.

So, if you're trying to improve the fuel efficiency, find a way to replace the carb with a fuel injector, as they did on the Suzuki engine. If you want more horsepower, then the supercharger is the way to go, assuming the engine can handle the increased pressure in the manifold and cylinder, the increased forces on the connecting rod, crank, and bearings, and the increased torque on the drivetrain components.

 

[Compositepro]

Electric turbochargers are commercially available and in active development. The ability to control them instantaneously to the needs of the engine has many advantages. Large truck engines will be first to benefit.

 

[kaelem]

For the power I’m Currently mid build waiting for a couple parts in the next few days, If I had to guess though I’d say it’d be pushing between 8-11bhp, and there’s a Rev limiter set at 10k(whether I reach it or not is a different story). And yeah I did mean crank as it’s a 2 stroke, I’ve seen a few that are commercially available but where most of them are eBay/Amazon products they seem to not to put dimensions often. The overall goal is improve the low end efficiency of the engine before the power band begins to hit, the bike had a relatively broad power band as it is (starting at 15mph and ending at around 45mph-some of the new parts I have will affect this so it’s all stuff that’ll need be reconsidered anyways)and boosting it into and through part of the power band I think would make it much faster on a take off(maybe wheelie easier too )

 

[BridgeSmith]

And yeah I did mean crank as it’s a 2 stroke,

Then I am lost on what you're talking about. My understanding of the components an internal combustion engine (ICE), 2 or 4 stroke, is that the crank is the eccentric shaft in the crankcase that rotates as it is driven by the connecting rod attached to the piston, which moves up and down in the cylinder, otherwise known as the combustion chamber. There are forces applied to the crank, but the supercharger compresses the air and fuel flowing into the cylinder, to be compressed and ignited, producing combustion. I think you'll find that terminology is common to most posting here. You may have to define the components if you're using different terminology.

The overall goal is improve the low end efficiency of the engine before the power band begins to hit,

What do you mean by efficiency? Are you trying to improve the torque, HP output, and acceleration (generally referred to as power output), or decrease the fuel used (typically referred to as efficiency)?

 

[kaelem]

I will try to be as specific and technical with my language, sorry for the confusion. In a 2 stroke the air fuel and oil mixture is pulled into the crank case to lubricate the crank, it then flows up channels (transfer ports) up the side of the cylinder to be combusted. If the piston is at TDC the transfer ports will be shut and can cause a spike in crank case pressure which could blow the crank bearing seals. The idea is improve the power output (the way I thought of it was if it’s working more efficiently it’d be making more power but I suppose that’s not technically correct). I want to improve torque and acceleration in the low rpm range 1500-5000. By mounting an ignition pickup (same mechanism as an ignition pickup but wired to the electric supercharger instead) on the stator slightly before the transfer ports open this should prevent a spike in crank case pressure and the supercharger will only be pushing air and fuel into the crank and then cylinder when the engine needs it too. I hope this clears things up, if not I may give up and read a dictionary

 

[waross]

And my other thought on this is that perhaps running it off of the ignition pickup which is pulsed it would only be compressing the air fuel mixture when the piston is about to come on a down stroke so as not to over pressurise the crank and blow the seals

Not to say that an electrical supercharger won't work, because it will work.
But you seem to have a massive misunderstanding of the energies and time lags involved.
There is a massive difference between the energy needed to make a spark and the energy needed to run a supercharger.
Yes it is possible to surround the flywheel with cores and windings and generate enough energy to light the lights and charge the battery and some small bikes do this.
You may still have an order of magnitude less energy available than needed to run a supercharger.
It may still be possible to use the flywheel to generate enough energy to run a super charger, but your flywheel most likely has one embedded permanent magnet. You will probably need about five more embedded magnets.
Do you have the machining skills to add magnets, not to mention the space.
Then you will have to surround the modified flywheel with laminated cores and windings.

On a budget of both dollars and skills, consider a battery driven supercharger for occasional use.
when cruising at low engine power levels, you may use the supercharger motor in regenerative mode and charge the oversized battery.
Then the supercharger may be kicked in to boost for short spurts of power.

Your concerns of pressure peaks at TDC.
Think about the dynamics.
The time required for the supercharger to spool up and develop a pressure spike will be many orders of magnitude greater than the short part of one cycle as the piston passes TDC.
And, as the piston travels upwards, the supercharger is delivering air to an expanding volume.
Past top dead center, as the piston descends, the reeds close and the supercharger is deadheaded.
The crankcase pressure increases without any input from the supercharger and peaks just as the port starts to open.

Think battery powered supercharger and we will look at ways to boost the charging circuit capacity.
How is the battery charged now?

ps; Battery power will probably give you the low end boost that need for wheelies.
Powering the supercharger from a generating circuit will have too much lag for much takeoff boost.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[kaelem]

Yeah learning about a lot of that ohms law stuff right now in college, the supercharger I’m looking at supposedly draws 5amps, when you say a battery do you mean the battery already in the bike or an external circuit? I had thought of something similar to using an external battery but was mostly concerned about the fact all the power would be going to one component and also how I’d keep the external battery charged. But yeah after you’ve said that I am really over complicating and over thinking this massively and just need to wire it in and see what happens I suppose, thank you for your patience and help I’ll let you know how I get on, got a top end rebuild to start tomorrow once it’s bedded I’ll start playing with the supercharger