What goes into making an efficient engine?

[user removed]

One of the most efficient engines on the planet has a bore of 3 feet and a stroke of 9 feet. Longer stroke works well for torque and horsepower at lower RPM.

In 1950 Alfa Romeo built a grand prix engine that was a 91 cubic inch (1.5) liter straight 8, which was actually two 4 cylinders bolted nose to nose. All accessory drives were gear driven off the junction of the two noses of the engines with two stage supercharging and 10k RPM red line.

390 HP at 2 MPG on the racing circuit.

It's like the difference between the old 225 slant 6 and a motorcycle engine that red lines at 15k RPM. The 225 had a stroke of 4.5 inches and a bore of 3.5. Lots of low speed torque. Geared right in a aerodynamic body with modern fuel delivery, the old slant 6 would still be competitive.

Best 5 speed gear ratios (overall) would give you red line engine RPM at 30,60,90,120,150 MPH in the respective gears. Considering a 6k RPM red line, that's 3k RPM at 75 MPH in top gear, just about exactly where the Civic VX final drive ratio was.

This doesn't seem to really follow the topic of the OP's thread which was about efficient engines which is a totally different subject.

regards
Mech

[Ecky]

Originally Posted by Old Mechanic

Best 5 speed gear ratios (overall) would give you red line engine RPM at 30,60,90,120,150 MPH in the respective gears. Considering a 6k RPM red line, that's 3k RPM at 75 MPH in top gear, just about exactly where the Civic VX final drive ratio was.

This doesn't seem to really follow the topic of the OP's thread which was about efficient engines which is a totally different subject.

regards
Mech

That's exactly how my car was geared then - at 75mph, in top gear, it's at exactly half of the redline of 8200rpm. The B16 would have been perfectly content purring along at about 2000rpm at that speed instead of 4000 though, and would have had power to spare.

[serialk11r]

The problem is it's hard to explain to the public a proper cruising gear because people will start wondering where the power went. "My engine has 240hp and I can't pass on the highway? WHAT A CRAPPY ENGINE!"
Meanwhile magazine reviews "the engine has amazing torque even in 6th gear!"

For performance driving, it is true that most 6 speed manual transmissions have a stupid 6th gear because the first 5 are the ones you use on a track and you hit top speed in 5th. When they talk "close ratio transmission" they don't mean evenly spaced, they mean the gear ratios are getting closer as you go higher! This is to counter the effects of air resistance I believe, since most performance driving happens at speeds where air resistance is playing some sort of role.

There is a place for more than 6 gears especially on an automatic because it will slightly improve acceleration, smoothness, and if you are cruising along at say 35mph, often times the last gear is too tall, so it's good to have a secondary cruise. On a manual transmission some people don't want to shift more often or don't understand the benefits, and other people just straight out don't care about fuel economy and want "response".

CVTs aren't used in performance driving because they have low torque capacity to weight ratio, and higher frictional loss, while the nature of a gasoline engine (tapering off power after a point) means you can have within ~85% of full power to play with at all times (usually over 90%) even with "just" 6 gears.

[hawk2100n]

Originally Posted by Old Mechanic

One of the most efficient engines on the planet has a bore of 3 feet and a stroke of 9 feet. Longer stroke works well for torque and horsepower at lower RPM.

In 1950 Alfa Romeo built a grand prix engine that was a 91 cubic inch (1.5) liter straight 8, which was actually two 4 cylinders bolted nose to nose. All accessory drives were gear driven off the junction of the two noses of the engines with two stage supercharging and 10k RPM red line.

390 HP at 2 MPG on the racing circuit.

It's like the difference between the old 225 slant 6 and a motorcycle engine that red lines at 15k RPM. The 225 had a stroke of 4.5 inches and a bore of 3.5. Lots of low speed torque. Geared right in a aerodynamic body with modern fuel delivery, the old slant 6 would still be competitive.

Best 5 speed gear ratios (overall) would give you red line engine RPM at 30,60,90,120,150 MPH in the respective gears. Considering a 6k RPM red line, that's 3k RPM at 75 MPH in top gear, just about exactly where the Civic VX final drive ratio was.

This doesn't seem to really follow the topic of the OP's thread which was about efficient engines which is a totally different subject.

regards
Mech

The info about stroke is dead on. It is one of the biggest things that an auto maker does. They have to start with the right tools to get the job done properly. If you at the latest civic, I believe they actually increased the stroke along with other numerous friction reductions in the engine to decrease consumption.

There are a lot of other things the designer can do to make an efficient engine. Gearing has been covered. But cam timing and variable phasing is responsible for big improvements. Also direct injection which allows higher compression makes the engine more thermodynamically efficient. Low drag accessories or electric operated ones like power steering and water pumps. There when you need them and off when you don't. Low friction coatings on the pistons and bearing surfaces; Teflon was one but newer compounds are common. Designing for lighter oils requires tighter tolerances and higher pump flow. Better fuel management and higher levels of atomization through high pressure direct injection and better nozzles. Better flow through the intake and exhaust. The list goes on but all of these things are areas that have been greatly improved in cars over the last 10-15 years and are responsible for reduced consumption of late but also maintain acceptable consumption in some of the very high horsepower cars you see commonly available.

[serialk11r]

Perhaps it's worth noting that increasing stroke increases friction slightly (part of the reason high rev engines use shorter strokes is to reduce both stress on the rotating assembly but also friction), but decreases the combustion chamber surface area to volume ratio and hence cooling loss, which is important if you want to run the engine slowly.

[user removed]

Longer stroke is a better "lever" for converting the pressure of combustion to rotational force at the crankshaft. The negative side is increased reciprocation forces, increased cylinder wall friction, and (as already mentioned) greater opportunities for heat loss to the cylinder walls.

It's basically the difference between torque and horsepower engines. The first gets its power at low speeds, while the second gets its power at high speeds.

There is another option not well understood by many and that is a modern derivative of the original aircraft rotary engine of WW1, a design that was long ago relegated to the scrap heap of history, but one that may have future applications, considering all of the modern advancements in metallurgy, fuel delivery, wireless communication, and lubrication.

regards
Mech

[SlideWRX]

My '07 mustang V8 can pull from *below* idle in fifth gear. I really don't need a fifth gear that works all the way down to ~20 mph. I would rather get more mileage. Right now first gear is short enough that I start from a stop 99% of the time in 2nd gear.

In the mustangs case as a drag racing vehicle, people at the drag strip (stock) run through third gear, and those that modify change the final drive to match the top of fourth to the speeds they are running. Fifth is still available to be a fuel economy gear.

In my case, the trans has a variant with a different fifth gear, (.62 vs my .675 ratio), and I have the optional 3.55 final drive versus the 3.31 base. I could also go aftermarket to a 3.27, 3.08, 2.73, or 2.41 if I could find them.

[some_other_dave]

Originally Posted by serialk11r

Perhaps it's worth noting that increasing stroke increases friction slightly...

That can be mitigated by using longer rods. However, you can run out of space for the rods inside the engine if you go longer. Rod:stroke ratio is an interesting subject...

-soD

[Autonomous]

Originally Posted by GRU

At highway speeds the mustang will probably be around 1500 rpm where he honda is probably around 3000 rpm.

Yeah my 3.8L v6 sits at 1450rpm at 90kph (60mph)