A work of pure genius! - Brilliant "Revetec" Engine

[santostripoli]

Isnt the issue here FUEL ECONOMY not performance orientation?

So from a fuel economy perspective, are we better having torque down low or up high???

[pneumatic]

One engine may be as efficient at 3,000rpm as another engine is at 2,000rpm. This gives the 3,000rpm engine the advantage as it can be geared lower. You see in order to get good efficiency from the vehicle, you'd gear it so you cruise at the most efficient rpm. One car would cruise at 3,000rpm, the other at 2,000rpm. Both getting the same efficiency.

But the engine that can cruise efficiently at 3,000rpm would be better. If both engines made say 100Nm of torque at their cruising rpm, the 3,000rpm engine would actually have 150% more torque at the wheels than the 2,000rpm engine (as it would be geared 150% lower).

When I was talking efficiency above, I was talking fuel efficiency. Try looking up brake specific fuel consumption. Engines have an efficiency point (curve/map really). Some engines have it low in the rev range, some have it higher in the rev range. If you cruise at a rpm lower or higher than the optimum, then you will use more fuel. So just because you are doing lower rpm doesn't mean you will be using less fuel. The rpm can be too low.

So there is no rule of thumb that says having the torque higher or lower is the best for fuel economy.

[revetec]

(as it would be geared 150% lower)

But in a sense the 3,000rpm engine would be like driving in third gear compared to the 2,000rpm engine in forth gear?

Pneumatic: yes it would have more torque at hand as you said at the roughly the same throttle opening.

But my experience of driving a car for almost 28 years, if I was to drive down the freeway in forth gear at 100kph and did a fuel consumption test and then did it in third gear it would use more fuel. If I was to do this the throttle would not be open quite as far but not 33% less. And let's assume we are driving a vehicle with a reasonably flat torque curve.

This is all good discussion but does it relate hand in hand with engine efficiency?

An engine does have a point where it is most fuel efficient, but if it is at 2,000rpm and another engine is more efficient at 1,000rpm higher say 3,000rpm with the same capacity etc, your throttle is slightly more closed in the case of third gear but not 33% less. If they produce the same torque at their individual fuel efficiency optimum points then the 3,000rpm engine would have to be alot more efficient to have the same outcome in overall fuel consumption when driven similarly wouldn't it?

This is all confusing but I know in real driving that when you drive down the freeway at 100kph in third gear you use more fuel than in forth gear, no matter how much torque you have at hand and where the peak is. In actual driving, as long as you have a good flat torque curve, it all comes back to good torque at a lower RPM is better for fuel economy doesn't it?

I agree with your comments on driving with a too low RPM. It is not good for overall fuel efficiency to operate an engine too far below the point when an engine has low torque and low efficiency, but let's assume that we're not doing this.

Did all that make sense? :-) Hopefully you get my drift.

[revetec]

BTW. Everyone is so concerned with peak power and torque when most of the driving occurs at a lower RPM range and at part throttle, and we normally look for good fuel economy especially on the freeway.

It would be good for car manufacturers to show power and torque figures at cruise or normal driving such as from say...10-30% throttle opening. This would give someone an idea of the engine's characteristics when driven normally.

I know this is a personal preference but I would much rather drive a vehicle with higher torque at a lower RPM because it is nicer to drive. I know there are many rev heads out there that like to give an engine a hard rev and I like to do it once and a while. But we are not looking for good fuel economy when we do this. When fuel efficiency is a concern then you don't hit the throttle as hard and you don't rev the ring out of it. Most people don't like to thrash their engine in day to day driving. Also higher RPM = more wear.

That's why I drive a 5.7 litre Caprice.....No old fart jokes please...I do enjoy a squirt of the throttle now and then. :-)

How many people operate their vehicle with their engine speed at the point of peak power and torque say 4,500rpm and 5,500rpm most of the time at full throttle when driving? I would probably imaging that you would operate your vehicle at these points less than 5% of the total time you drive wouldn't you? I am refering to an average driver, not an 18yo or race car driver ok?

[pneumatic]

That's why I drive a 5.7 litre Caprice.....

That is the reason why your perspective is somewhat distorted. A big V8 is more efficient at low rpm, they are torque monsters

I drive a 1968 VW Baja, with a 1992 Mazda MX6 quad cam V6 in the back (I built it myself). It makes peak torque at about 5500rpm, peak power at 6500rpm and revs to 8000rpm. Now it is a high performance motor that revs very hard, but it's also very very torquey. It's torque curve is relative flat from 2000rpm to 6000rpm. I drive it offroad and it will happily crawl around at 500rpm (seriously!). I put a custom 4th in it so it cruises on the highway at about 2250rpm to get good economy. So just because an engine is revvy and produces torque at high rpm, doesn't mean it can't produce sufficient torque at lower rpms to move the car along and get good low rpm efficiency.

Besides, not all engines are the same. Not all engines like cruising around at 2000rpm (or less). Most small (and efficient) cars are designed to cruise at highway speeds at around the 3000rpm mark. They wouldn't accelerate and would get bad economy at 2000rpm. Just like your V8 will get bad economy revving at 3000rpm, past it's efficiency point. My motorcycle is designed to cruise at 4,500rpm on the highway, this gets good economy and is how it's designed to run.

So what I am saying is that where the peak torque is has absolutely nothing to do with how fuel economic the engine is or will be, or even at what rpm it will be most fuel economic.

The throttle opening can also not be compared in the way you are talking about. If it takes 30HP to push a car through the air and travel at 100kph, then it takes 30HP no matter what gear your in (ignoring the different mechanical losses). Think of the throttle butterfly as a flow orifice. 20% throttle opening might relate to flowing 30HP worth of air. This will be the same for any gear. The problem is you need to take into account the higher mechanical losses (inertia related) at higher rpm, and also the effect of the manifold vacuum changing (which changes the volume of air flow for a certain restriction size). So basically the relationship between throttle opening required to maintain speed and rpm the engine is at is quite complicated and not a simple relationship. It isn't simple enough to make the kind of comparisons that you were thinking about.

[manolis]

The image below is made from some results and plots created by “Pattakon RoadLoad DOS program”.

It is for a two stroke Kawasaki 125cc and for a RHL4 Revetec propelling a typical family car.



To download the Pattakon RoadLoad program, create a folder in your computer and save there the files you see in the
http://www.pattakon.com/educ/roadload folder.
With this program it is easy to find, among others, the best gearbox ratios, the revs of best gear shift, the expected best possible performance etc etc.


Kawasaki 125 has plenty of torque (compared to its capacity) at extreme revs. But if you leave the revs of maximum torque, it is ‘chaos’.

Beyond the maximum torque and beyond the revs at which the maximum torque is provided, the distribution of the torque is also – if not more – important. Furthermore, the torque curves at partial loads (partial load response) greatly affect the drivability of the vehicle (the typical torque and power curves are with the throttle valve wide open, i.e. 100% of the load).

The fuel consumption is not directly related to low revs peak torque, as many comparative drive tests show. I.e. things are more complicated than just saying “low revs torque guarantees low consumption”.

The best is to have an engine with flat torque (and good response at partial loads) along a wide range of revs, from let say 800 to 10,000 rpm.
The max-min ratio 10.000/800 gives a good idea for the drivability of the vehicle.
A calculation of the max-min ratio of the engines of this discussion would be interesting.

Thanks
Manolis Pattakos

[revetec]

This theory all looks and sounds good but......

I drove my car last night showing the instantaneous fuel usage on my trip computer. I picked the rpm band that the torque of my engine is in it's flatest point. I maintained 100kph in fourth gear and the trip meter said 11.9 litres per 100kph. I then put it in third gear and travelled the same stretch of road and maintained the same road speed and the fuel consumption was reading 12.8 litres per 100kph.

Maintaining the same speed in the same vehicle would make you assume that the same work was being done so the power usage would be similar, with a 2% variation from the extra transimission losses. Wind and rolling resistances are the same. That's a 5% fuel saving in 4th gear.

Can anyone else do this with their vehicle and post results?

Comments for?: I was consulting with a chassis dyno specialist and he said that if a factor of 12-15% transmission losses are to be factored in for a front wheel drive car then the test must be carried out in third gear. If you use a 1:1 ratio (usualy forth gear) as many dyno testers use, then the transmission has no gearing losses as such. There are final drive losses which are a constant and there are of course is losses from driving all the transmission shafts around. Any comments on this subject as I'm interested if any have you experienced tests and whether they were carried out in 3rd or 4th gears?

[pneumatic]

With this program it is easy to find, among others, the best gearbox ratios, the revs of best gear shift, the expected best possible performance etc etc.

My Excel program does exactly the same thing, but is a bit easier to use (DOS programs aren't very user friendly ).

The fuel consumption is not directly related to low revs peak torque, as many comparative drive tests show. I.e. things are more complicated than just saying “low revs torque guarantees low consumption”.

That is exactly what I was saying!

Maintaining the same speed in the same vehicle would make you assume that the same work was being done so the power usage would be similar, with a 2% variation from the extra transimission losses. Wind and rolling resistances are the same. That's a 5% fuel saving in 4th gear.

Can anyone else do this with their vehicle and post results?

Brad, as I said above, you are only looking at your engine which has it's most efficient point low in the rev range. So of course if you operate it above the most efficient point it will use more fuel. But you have to understand not all engines are big lazy V8's. Most small fuel efficient cars will need to sit closer to the 3000rpm mark to be the most efficient.

But as I and Manolis have said, the point where a car makes peak torque doesn't necessarily equate to the point where it is most fuel efficient. Your car is the perfect example. A 5.7L Holden Caprice makes peak torque at about 4800rpm!!! I am sure your engine wouldn't be very fuel efficient at that rpm.

So basically the torque curve of the revetec isn't saying anything about whether it will be more fuel efficient than another engine with it's peak torque at higher rpm.

You cannot say that just because the revetec has torque low in the rev range, that it will get good economy.

If you use a 1:1 ratio (usualy forth gear) as many dyno testers use, then the transmission has no gearing losses as such..... ....Any comments on this subject as I'm interested if any have you experienced tests and whether they were carried out in 3rd or 4th gears?

I think your dyno operator is getting confused by some transmissions (only some rear wheel drives I think) that have an exact 1:1 gear that actually locks the input shaft and output shaft together (bypassing the countershaft and not running though any gears in the gearbox itself). So this makes a perfectly efficient gearbox, but it still has to go through the diff (which is inefficient).

For those that are interested, there are two basic types of dyno's, inertial and load types. Inertial are usually the cheapest and are more prone to the effects of gear selection, because each gear multiplies the drivetrain inertia differently (a square of the ratio).

A dyno dynamics dyno is a load type dyno and is more accurate. The company has also made what they call "shoot-out" mode. They have made it as an industry standard method of testing HP, so no matter which dyno you test the car on you get the same results (or so they say).

The load type dyno is still effected by drivetrain inertia but not as badly. The operator will enter the vehicles drivetrain inertia from a standard list of vehicles, however if the drivetrain or engine is modified the figure will be slightly out. This can lead to a small difference in HP figures between gears.

Some gears are also slightly different in efficiency than others. Generally the lower gears like 1st and 2nd are coarse teeth gears for strength, and the higher gears have finer teeth for efficiency and quieter operation.

So dyno guys will dyno the car in the highest gear possible, without exceeding the max roller speed. The max roller speed on most dynos is between 200-250kph, so third gear on most cars is normally used. Using low gears means you notice the effect of the drive train inertia quite a lot, but it also means you have more chance of wheelspin on the rollers.

A gear set only has a certain efficiency for transmitting the power. Spur gears are the most efficient (but cars don't use them because they are noisy, but race cars do use them because they want the efficiency). Most cars have helical gears. These produce a thrust load which is inefficient is wastes power.

You do notice a difference between front wheel drive and rear wheel drives. This is because most rear wheel drives have hypoid diff ring and pinion gears. These are very inefficient, but quite strong. Front wheel drives just have another helical gear which is more efficient that the hypoid.

[revetec]

I know that front engined/rear wheel drive cars have a 1:1 gear ratio, usually 4th in a manual and 3rd in a 4 speed auto.
Front wheel drive cars usually don't. You're right by saying hypoid are not as efficient as helical, and spur gears are even more efficient. I know that your VW baja doesn't have a 1:1 ratio in any gear either.

Gear ratios and efficiencies change with each vehicle and it also changes with load, so all this shows that a chassis dyno is just a guide unless you know exactly what the drive train losses are under all conditions.

[revetec]

I will try and get some FWD & 4WD ratios....

[pneumatic]

Just be careful, that table above may be a little misleading.

Just because a gearbox has a 1:1 ratio doesn't mean it is the type of 1:1 ratio that locks the input shaft and output shaft together (to get the good efficiency).

The VW transaxle I have has the input shaft and output shaft in different planes (not inline). Therefore you cannot couple the two together.

Front wheel drive transaxles have the same problem, the input shaft and output shaft are in different planes and cannot be coupled together.

It is only possible for rear wheel drive transmissions that have the input shaft and output shaft inline with each other. And even then it doesn't garantee that it has that type of locking shaft design. If it has this feature it is termed "direct drive", and if it doesn't it is termed "indirect drive".

So you might think a "direct drive" gearbox would be the best option, but this isn't necessarily so.

You see a direct drive gearbox goes from an input shaft, to a counter (or lay) shaft through a pair of gears, and then back from the counter shaft to the output shaft through another set of gears. So for all ratio's other than 1:1, they have the efficiency loss of going through 2 sets of gears.

An indirect drive gearbox has an input shaft and an output shaft that aren't inline with each other. Therefore the power only ever goes through 1 set of gears, so will be more efficient in every gear other than the direct drives 1:1 gear.

[revetec]

www.fueleconomy.gov

Use Overdrive Gears

When you use overdrive gearing, your car's engine speed goes down. This saves gas and reduces engine wear.

Estimates for fuel savings from sensible driving are based on studies and literature reviews performed by Energy and Environmental Analysis, Inc., Washington, DC.

Quote taken from the EPA....

If you have a manual transmission, the lower the shift speed, the better the fuel economy.

Quote taken from Ezine Articles

Use Higher Gears: This might not be of particular importance to those driving automatic transmissions, but getting a car up to cruising speed and putting it in the highest effective gear available uses a lot less fuel.

Quote from MSN

Use a high gear or overdrive when driving at highway speeds. The high gear will reduce engine speed, saving gas and cutting engine wear.

Quote NBC

Get A Manual-Shift Car: Manual-shift cars allow you to change to the highest gear as soon as possible, saving gas.

Everyone else seems to agree on the point that the taller the gear...the more fuel you save (as long as engine RPM isn't too low and the engine is labouring).

Quote from Pneumatic:

If you cruise at a rpm lower or higher than the optimum, then you will use more fuel.

My Quote from before:

I drove my car last night showing the instantaneous fuel usage on my trip computer. I picked the rpm band that the torque of my engine is in it's flatest point. I maintained 100kph in fourth gear and the trip meter said 11.9 litres per 100kph. I then put it in third gear and travelled the same stretch of road and maintained the same road speed and the fuel consumption was reading 12.8 litres per 100kph.

Maintaining the same speed in the same vehicle would make you assume that the same work was being done so the power usage would be similar, with a 2% variation from the extra transimission losses. Wind and rolling resistances are the same. That's a 5% fuel saving in 4th gear.

[pneumatic]

Everyone else seems to agree on the point that the taller the gear...the more fuel you save (as long as engine RPM isn't too low and the engine is labouring).

I agree with that too, which is why I said "If you cruise at a rpm lower or higher than the optimum, then you will use more fuel" (as you quoted). No-one is debating any of those statements.

The point is;

The most fuel efficient cruising RPM is NOT related to where an engine makes peak torque.

AND

Cars have different top gear ratios depending on the engine and vehicle. Some can be very low like 1,500rpm, others can be quite high like 3,500rpm. Motorcycles even higher...

[manolis]

Pneumatic,

the RoadLoad program is for those who have not your excel nor your background, especially for the youngsters.

Here is a plot for your discussion (I have ‘stolen’ the basic BSFC plot from internet and added the rest).



Thanks
Manolis Pattakos

[revetec]

Pneumatic: My point is that if you produce better torque in a lower in the rpm range (which will lower the point in RPM when the engine labours) then you can use taller gearing which then saves fuel. A performance engine which has a higher RPM torque and power band labours at a much lower RPM.