How much improvement in power output can I expect if in an engine of 20mm bore and 17.5mm stroke I move for a simple flat head/flat piston to a hemispherical design?
I can see how the shape of the piston /cylinder head profile might affect a large cylinder, where there is a large amount of gas to move, but in a small one? It's a lot of extra effort in the making if the performance improvement is debatable?
It allows bigger valves for a given bore size, unshrouds them because they open away from the bore walls and as you incline a valve from the vertical it automatically makes the ports downdraught rather than horizontal with a
90 degree bend so the port efficiency and airflow is much higher. Getting the air round that 90 degree bend is the single biggest limiting factor with horizontal ports.
With vertical valves you obviously need a bit of clearance between each valve and the bore wall and also between the two valves. If you push things to the limit you can get the total diameter of the two valves up to about
95% of the bore diameter and the rest is used up in clearance. With inclined valves in a hemi chamber, to give you a real example, I can use a 45mm inlet and 37mm exhaust valve in a Ford CVH engine with an 80mm bore and that's not quite the bitter limit. The total valve diameter is 82mm i.e. 102.5% of the bore diameter.
That's an advantage of 102.5 / 95 = 108% on diameter so 16% extra valve area in a given bore size plus as I say inclined valves tend to flow more air anyway - perhaps an extra 5% to 10%. The downside is a worse combustion chamber shape with negligible "squish" area which tends to burn rather slowly plus some extra manufacturing complexity. You can't have a single overhead cam with flat bucket lifters for example. Rockers are needed with a single cam or you have to use two cams. High output Porsche engines use twin spark plugs to help get round the combustion chamber shape problem and this makes a significant difference to power compared to single plugs.
Netting everything out I'd say the potential advantage of a hemi design in power terms is about 15% to 20% if everything is done right. Without twin plugs it would be less. If this is your first engine I'd stick with simple for now though. Concentrate on trying to design the ports as downdraught rather than horizontal and you'll recover some of the disadvantage of vertical valves. Even 20 to 30 degrees of downdraught angle will give you a much bigger radius short side turn in the port and better flow. Pure racing engines aim to get as much downdraught angle as possible so the air has a straight shot right through to the valve seat.
I have a sneaking suspicion that with a 5.5cc swept volume and a 4.2cc head, the extra surface area/volume caused by a hemispherical head and piston to match, might lead to poor combustion with this size engine.
There's scope here for a nice long series in Model Engineer with lots of variations in design :-)
umm, for us old fa*ts that's about 3/4" bore. Improved fluid flow may give you more power, but the ports must be angled to improve swirl. pinging/pinking is no problem in a bore this small. An easier solution might be to use a modified squish combustion chamber. high compression ratios are easy to achieve. Port flow not so good, as they must be smaller, but swirl is excellent.
May I suggest getting the two volume set "The Internal Combustion Engine In Theory and Practice" by Taylor MIT press. A good read even if you can't handle the calculus.
It's my second (first one is on Youtube but the video isn't great, I was struggling to man the engine and the camera).
This one I want to make use of, so the ambition is not just something that will run, but will run well (ambition is to stick it in an RC plane). It's a V-twin, one simple carb per cylinder. So I'll take your advice and make the Inlet valve as big as it will go and move inlet port from 90degrees to a shallower angle. That's probably enough complexity to add to the existing plans. I'll report back in a year or so...
Reminds me of a Renault 16 TX I once owned. That had a hemi head, but the cam was in the block and it had twin rocker shafts. The engine was light alloy and featured wet liners - light years ahead of the stuff available from Ford and BMC like the Kent and A series. ISTR it made about 93 or 98bhp from just over 1600cc, and lasted well over 100,000 miles - not bad for a car engine from the seventies.
I well remember my TR3A reg 354 KPH amazing how thats the only reg I ever remember apart from my current vehicles. Took it home in 3 trips with a Landrover, all in bits. Rebuilt and resprayed it myself, Magic motor! All at 19 years old in fathers "small" garage. Bought for £100 sold for similar when I got a company car a few years later.
The TR3A and the MGA were the last of the great sports cars IMHO (ok and the Healey 3000). Wish I still had it I would feel 20 years younger
I miss my old 57 TR3. Bought it not running in the late 1960s. Souped up the engine, repaired an rebuilt everything. What a fun car. One spring I drove it from Victoria British Columbia to Montreal Quebec. Three thousand four hundred miles each way!
Looking at the plan again, the fuel/air mix from the carb makes a hard right into the cylinder head and then there is another 90 bend into the cylinder. So if I get rid of one 90deg turn and soften the other to 45 or better, that should make a material difference to the performance?
619SRF, it cost me 125 in running order. I liked it so much that I owned a TR for 44 years. Sold the last one last year, pretty much in showroom condition, I just didn't dare risk leaving it in a car park or driving in heavy city traffic.
Vauxhall Astra van these days: it does go round corners with the back end following the front. Warm inside on a cold day, too.
I miss mine too, but its gone to a good home. Wondering about a Lotus
7 or Caterham for next summer.
I have some TR2&3 spares if anyone here needs any.