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Old 14th January 2011, 16:46   #1
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Question Truck Diesels vs. Car Diesels

Why are truck diesel engines having a high displacement and sticking to 6 or 8 cylinders in a straight line? Diesels in cars have cylinders in V orientation and some even make more power from a lesser displacement than say the Volvo 9400s.

Why is this so? Is it simply for torque?
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Old 14th January 2011, 17:31   #2
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Default Re: Truck Diesels vs. Car Diesels

In a truck space is not at a premium. Engines can be designed totally different.
Also you have to appreciate that trucks can be as heavy as 30 luxury cars.

An inline engine is easier to balance than a V configuration. With the extreme mass of big Diesels this has got a heavy impact.

In some areas commercial Diesel run between 1,20,000 to 1,80,000km a year carrying a lot of weight. These engines need to last at leas 10 lakh km.

This can only be done with lowe revving engines that have extreme torque outputs. Additionally the low rpm is good for fuel economy.

In passenger cars it is a bit of a different story. The engines have to have a wide torque range, good torque and have to be efficient without losing comfort for the driver. This ask for a different approach.
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Old 14th January 2011, 17:49   #3
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Default Re: Truck Diesels vs. Car Diesels

Quote:
Originally Posted by CPH View Post
In a truck space is not at a premium. Engines can be designed totally different.
Also you have to appreciate that trucks can be as heavy as 30 luxury cars.

An inline engine is easier to balance than a V configuration. With the extreme mass of big Diesels this has got a heavy impact.

In some areas commercial Diesel run between 1,20,000 to 1,80,000km a year carrying a lot of weight. These engines need to last at leas 10 lakh km.

This can only be done with lowe revving engines that have extreme torque outputs. Additionally the low rpm is good for fuel economy.

In passenger cars it is a bit of a different story. The engines have to have a wide torque range, good torque and have to be efficient without losing comfort for the driver. This ask for a different approach.
There are 3 liter engines in cars making ~500 Nm of torque at 1500 rpm or so.
What kind of figures are we looking for an average truck diesel?
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Old 15th January 2011, 05:02   #4
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Default Re: Truck Diesels vs. Car Diesels

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Originally Posted by smashnerd View Post
There are 3 liter engines in cars making ~500 Nm of torque at 1500 rpm or so.
What kind of figures are we looking for an average truck diesel?
Its not only figures on paper that do that job. Where, and how it is delivered across the rev range determines drivability/practicality of the engine.
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Old 15th January 2011, 08:57   #5
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Default Re: Truck Diesels vs. Car Diesels

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Originally Posted by dhanushs View Post
Its not only figures on paper that do that job. Where, and how it is delivered across the rev range determines drivability/practicality of the engine.
Could you elaborate a bit more on the "how it is delivered in the rev range"?
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Old 15th January 2011, 11:11   #6
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Default Re: Truck Diesels vs. Car Diesels

Truck engines are low revving (very rarely rev. to 3500rpm). PD is common (noise is Ok), and valve lifters/cams are mostly rotary. All in the interest of a long life and low consumption.
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Old 15th January 2011, 13:26   #7
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Default Re: Truck Diesels vs. Car Diesels

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Originally Posted by smashnerd View Post
There are 3 liter engines in cars making ~500 Nm of torque at 1500 rpm or so.
What kind of figures are we looking for an average truck diesel?
Engines for smaller commercial vehicles start at about 70Nm. Bigger trucks vary from about 1200 to over 4000Nm.

The torque requirement depends on a number of conditions, where terrain is one of them. If you have a 44 ton truck and have to cover rugularly level differences of many hundred meters much more torque will be needed or you never will get anywhere. At the same time you need to maintain fuel economy.

Quote:
Originally Posted by smashnerd View Post
Could you elaborate a bit more on the "how it is delivered in the rev range"?
a modern 5+ liter truck engine will not even rev beyond 2,000rpm. and may tick over as low as 200rpm. These engines need shifting for optimal fuel econmy at about 1200rpm the latest and usually cruise around 900rpm.

The turbo comes in soon above tickover. It is fairly easy with modern ECU control and VGT turbos to a low lag performance, which is aided by the narrow RPM range of big engines. Additionally todays gearbox technology helps where the semi automatic gearboxes select the best rpm for the individual application. These gearboxes start with 12 gears and might have more than three times as many gears.

Last edited by CPH : 15th January 2011 at 13:38.
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Old 15th January 2011, 16:41   #8
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Default Re: Truck Diesels vs. Car Diesels

Quote:
Originally Posted by sgiitk View Post
Truck engines are low revving (very rarely rev. to 3500rpm). PD is common (noise is Ok), and valve lifters/cams are mostly rotary. All in the interest of a long life and low consumption.
What's PD? And what do you mean valve lifters/cams are mostly rotary?
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