two stroke lesson?

[ckone0814]

Bear with me, trying to learn here...

As I'm looking at various used boats for sale I'm obviously considering the power. Four stroke?...got it. Two strokes?...I thought I had it.

I (incorrectly?) assumed that there were two types of two strokes; carbed and DI but two vessels for sale had motors that were 2S DI but when I looked at the manuals and parts lists online I see carbs.

So I'm confused.

Are there different types of 2S injected? Maybe DI and OI are not the same as I thought?

Can someone give me a brief lesson? If it's not brief, can someone point me to a good website or something?

 

[rodbolt]

Re: two stroke lesson?

DI simply means the fuel is injected Directly into the cylinder after the piston has closed off the air intake and exhaust ports. optimax,HPDI,and E-TEC are some examples.
EFI simply means that instead of carbs infront of the reed valves we have injectors that spray a consistant amount of fuel based on inputs to the engine control computer and outputs for injector on time and on some fuel pump speed.
then you have the oddball Tohatsu TLDI which is still a direct injected engine just works slightly different.
fuel injector rail pressures for EFI are typically about 35 to 45 PSI, for optimax I belive its about 110 but may be wrong,for 2.6L HPDI its roughly 650 PSI and for the 3.3L HPDI its about 1100 PSI.
so we go from relativly low tech carbs,to higher tech EFI to the latest in two stroke tech the Direct Injected versions.
what this allows is more complete combustion with less emissions or scavenged fuel blowing out the exhaust.
it also gets leaner and leaner and while carbs may tolerate some degradation the DI motors typically dont.

 

[ckone0814]

Re: two stroke lesson?

Thanks a lot. I have the basics of what your saying but how does a carb figure into a DI two stroke? Isn't everything injected into the cylinder? So hows a carb in there?

 

[rodbolt]

Re: two stroke lesson?

you selected the wrong diagram in the manual.

 

[Silvertip]

Re: two stroke lesson?

I think what you are assuming are carburetors are merely the throttle valves. They simply meter air and no fuel actually passes through them. When cars in this country first utilized fuel injection, they were indeed called throttle body injection which later evolved into port injection where fuel was injected into the manifold runners but the throttle body was still there as it is today. Regardless what type of "injection" you have, some form of air metering (throttle setting) is necessary.

 

[ckone0814]

Re: two stroke lesson?

Still reading and i think I got it...

I'm confusing OI and DI.

OI just means no mixing oil and gas, yet still carbed? DI is FI, "e-tec" technology, no carbs?

I had a Yami 115 4 stroke on my first boat (owned with partner) but have since sold out my half. Now looking for myself and have to look at older boats and technology which brings a lot more questions. Maybe I should stick with a 12' Grumman with a left oar and right oar...

 

[coolguy147]

Re: two stroke lesson?

then if you get a 2 stroke engine with carbs there are 2 types


the much older crossflow and the newer more efficent looper. both drop fuel in the water. the newer ones just not so much by a big difference then your older i mean way older crossflows

 

[JB]

Re: two stroke lesson?

I think you, or someone else, is confusing oil injection with fuel injection and/or direct fuel injection.

If it has carb(s) it is not fuel injected or direct injected, but it may be oil injected. EFI and DFI engines are also oil injected.

Rodbolt's explanation is spot on.

 

[kenmyfam]

Re: two stroke lesson?

Bang on there Rodbolt. No further explanation needed in my opinion.

 

[ckone0814]

Re: two stroke lesson?

you selected the wrong diagram in the manual.

Nope. Had the right manual and diagram. As others and I have said, confusion between OI and DI. Thanks.

 

[Cricket Too]

Re: two stroke lesson?

If you definitely had the right manual, then what you think are carbs, are the air plenum intakes, or throttle valves, as was mentioned above. Even if you inject the fuel, in any way, you need to have air coming in, same as on your fuel injected car engine. Engines are basically air pumps when it comes down to it, with some fuel added into the air to make it ignite and explode under compression.

I think why you are confused with OI and DI, is because they are two completely different things. I wouldn't concern yourself with OI, if you are just concerned with the fuel system types and how they work.

DI means the fuel is Direct Injected into the combustion chamber, close to the end of the compression stroke, after the air has already been pulled in and almost fully compressed. EFI means the fuel is injected into the throttle body (AKA throttle body injection), and the fuel and air charge is pulled in together and compressed, similar to a carb.

OI means the oil is injected into the engine separately from the fuel. Sometimes it may also be confused with OMS (VRO), which mixes the gas and oil before they are drawn into the carbs. I have heard this called oil injection before, but oil injection is a separate thing.

In any case, the 3 types of 2 stroke fuel induction methods are Carb, EFI or DI.

If you have a carbed 2 stroke, it will either be a pre-mix, where you mix the gas and oil manually in the fuel tank, or it will be mixed by the engine and pulled into the carb mixed (OMS/VRO).

If it's and EFI 2 stroke, the fuel will be injected into the intake air charge in front of the intake manifold, and the fuel and air mixture will be pulled through together.

If it's DI 2 stroke, the fuel will be injected directly into the combustion chamber, usually on the opposite end of the engine from the air intake (back of the engine).

OI and DI have nothing to with one another, it's not one or the other, but you could have both, as layed out by JB.

Hope this helps, I may have started rambling.