RAM 1500 Diesel Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
677 Posts
Discussion Starter #1
Mazda has put a diesel into the CX-5 SUV. Redline is a big deal for diesel engines because hp is proportional to rpm. So if an engine can rev 30% higher without losing much instantaneous torque, you get 30% more hp. That's why diesel engines aren't very powerful....low rpm. Remember, torque isn't power because there's no "motion" nor "time" in torque. The magazine writers aren't engineers so they don't understand that basic idea. Power is power, and more rpm is more power. So dang, 5500rpm from a diesel. That's cool. Wonder how they did that.

https://www.cnet.com/roadshow/reviews/2019-mazda-cx-5-signature-diesel-awd-review/?
 

·
Registered
Joined
·
4,500 Posts
on a simple displacement ratio a couple things can be learned. Proportionally, each cylinder is bigger than ours, torqu is proportionally less than the ecodiesel but hp is proportionally more. I didn't have time to compare bore and stroke but it could be as simple as tuning for a rpm and horsepower bias and not so much for economy. The economy wasn't particularly impressive. I hope they succeed. Will be interesting to see if they lower the trim level the engine is available with if the uptake isn't high enough on the upper trim level.

I have always thought Mazda had some brave engineers and management willing to take some risks. Will be interesting to see how it goes for them.
 

·
Registered
Joined
·
3,901 Posts
Ive seen a 6.0 powerstroke over 5500 rpm. I wasn't present when it blew but it did.

My understanding is the rpm has less to do with the fuel and more with the design of the engine valves etc.

It may not be perfectly accurate but i was taught torque is how much work a motor can do and HP is how fast it can do that much work.
 

·
Registered
Joined
·
12,637 Posts
Thanks for the link to make us aware of this thing. "Thing" is my description. High rpm and low fuel mileage combined with high cost and whatever emissions issues they have. The "thing" barely gets Ecodiesel fuel mileage with a teeny car.

Volkswagen had a "Thing" that today is a sought-after classic. This thing is a loser.
 

·
Registered
Joined
·
1,484 Posts
With some head work and better valve springs our ecodiesel could spin 5500 rpm just fine. It wouldn't make much if any more power without a bigger turbo and possibly a bigger common rail pump but it might.
 

·
Registered
Joined
·
12,637 Posts
Always look at more power to mean you need - more truck, more motor, beefier payload, more towing capacity etc.

All of that is available in all the 2500 diesel trucks available today. To ask a little 3.0 diesel engine to make the power of the big boys in a light-duty component pickup is not realistic. Reminds me of the kids that raise up a 1500, put on the big tires and try to look like a big truck. Stick a load on them and the frame might sag down like a rented mule.
 

·
Registered
Joined
·
785 Posts
Years back, I worked with a young kid that bought a lifted F150, might have been an F100 at that time. The first time he shifted it into 4 wheel drive, he sheared the transfer box. Found out his lift was about six inches too high.

To OP: It's my understanding, from my VWs, that the power is torque. If you look at the torque curve, once you start down the backside, you are no longer adding power. The flatter the top of the curve, the longer you can hold power. My VWs peak around 1800-2000 rpm. Once you're past about 3500, you fell the engine fall flat. I've not looed for a torque curve on the ED engines but am speculating that you'd see the same thing. If Mazda is running diesels to redline, it's hard for me to imagine that the torque curve is continuing to climb through the full 5500 rpm.
 

·
Registered
Joined
·
91 Posts
a quick engineering lesson:

The definition of Work is Force times distance, W=Fxd; and Torque, measured in units of pounds times feet (lbsft, or ftlbs, both are acceptable), is by definition Work. so there is motion in Torque, but it's rotational not linear.

The definition of Power is Work times time, P=Wxt. (note, lowercase t is time, Capital T is Torque)

So, Horsepower and Torque are directly related by the rpm of the engine, and the equation is HP=(TxRPM)/5252; where 5252 is a constant required to convert units of Torque and RPM to HP. So as RPM increases so will Horsepower (up to a point).

Engines do not make Horsepower, they make Torque; and you will likely never see the Horsepower that your engine is rated for because you will never constantly run it at that RPM. For this reason I generally ignore Horsepower ratings for the practical use of engines.

For automotive engines, mechanical design of the valves, valve springs, rockers, cam, crank, rods, pistons, etc. will govern the max RPM of an engine, as well as the torque made throughout the RPM range. I believe most redlines are governed by the valvetrain; either the valves aren't big enough, or the cam doesn't have enough lift, or you experience valve float (where the springs can't act fast enough to close the valve completely before it's opened again). It could be possible (though not likely) that you exceed the strength of the bottom end. Crankshafts that have a long stroke, or are unusually heavy, can't spin as fast. Engines are designed so the piston moves up and down at a certain speed (mean piston speed), and exceeding that speed (higher RPMs) can cause bearing failures, rod failures, piston failures, etc.

If all of those issues can be solved however, your limiting factor will be flame propagation, or how fast the fuel burns. This is not typically an issue in gasoline engines, mostly because gasoline combustion is extremely fast but also because nobody makes really large gasoline engines. Flame propagation is a major factor in diesel engines because the fuel burns so much slower. This would not typically be an issue in the ED or any automotive diesel in use today, but in large commercial medium and low-speed diesels it's commonly considered in the engine design. For example, the largest diesel engine in the world, the Warsilla Sulzer RTA96-C, has a redline of 200 RPM, mainly because the time it takes for one combustion event in one cylinder can be measured with a stop watch (this is a supposition and/or exageration on my part, i don't really know the combustion time but I know it's long enough to affect max rpm).
 
1 - 8 of 8 Posts
Top