Hydraulic Hybrid Truck Conceptual Design

Tom

Marcin;

My thoughts were to start with an existing truck, like this one for $1000 with blown transmission:

https://houston.craigslist.org/cto/d/alleyton-2000-gmc-w3500-forward-box/7898859919.html

Replace the transmission with one hydraulic pump and one hydraulic motor to power it via the existing engine and differential. This would be a simple way to design the hydraulic system apart from the rest of the truck.

Of course, the power cube / 4 wheel drive thing sounds attractive, but I'd like to focus on getting the hydraulic drive train working smoothly before going further. It did suggest the following for 4 wheel drive:

Layout B — Four Motor Wheel Drive Pump: Parker / Eaton variable pump in 110–150 cc range

Motors: 4 × mobile wheel motors (100–120 cc)

Multiple closed-loop hoses

Traction control via independent motor swash controls

One thing I'd like to defer for now is integrating the 4 wheel drive and steering. Using an existing truck's steering, braking and so on is preferable for the first cut.

The prices gathered seem too high. I think alternatives / other sources can cut it substantially. It did say other pumps could work: Best hydraulic pump options for truck propulsion (ranked):

Rexroth A4VG series — ideal overall

Parker/Denison T6 series — excellent reliability

Eaton VP/PVQ series — good cost-performance

Vehicle-specific hydrostatic transmissions — highest integration

Besides, SurplusCenter sells Rexroth A4VG pumps for $800 and they sell Rexroth motors as well:

https://surpluscenter.com/piston-pump-a4vg28hwdl1-32r-nsc10n005e-s/

The beauty of the variable displacement pump is that the throttle connects to the pump and controls it's output with a feedback loop inside the pump for reliable, well controlled output.

I'd like to maximize the safety and prevent the possibility of it behaving erratically. It would be best if it works like a normal truck as much as possible.

MJ

Challenges of speed and power control for a truck with open center hydraulics: https://chatgpt.com/share/69480f12-1ea0-8010-a3cc-15a351f4209a

Please post source chat at https://wiki.opensourceecology.org/wiki/Open_Source_Hydraulic_Truck

You forgot engine and transmission in your cost estimate.

4500 lb truck is a sissy, need to pull real loads. Need 4WD.

$8k-$14k for hyd pump/motor is too much. You're now in a new class of more complex hydraulics than simple power cube stuff we did to date. Point would be to get rid of transmission. Use hydraulic motor on each wheel.

Hydraulic pumps (nonvariable) are $10/hp. Use engine throttle or multiple engines (power cubes) for speed control, not variable pumps. If you control pump through variable feature then engine is revved way up, which is not efficient. Huge efficiency would be say 1 engine on low speed, 4 engines on high power - with ability to turn on each engine as needed. 

I don't have a good affordable solution for engine. I would go with 4x 1000 cc but that is only 152 hp. We need a larger engine that is affordable.

What is the best we can do?

How about a heavy engine from caterpillar and no emission design - for off road DIY only - and as we sell some bootstrap to full emissions and our own engines?

I would do this phasing: 

1. Robust, cheap - no emissions. Work out robust speed, power, scalability with open center hydraulics.2. Invest in CNC centers, induction, and forge - make our own engines3. Develop open source high performance hydraulics.

The only way I can think of good speed control:

1. Lo/hi throttle with 4 power cubes - gives equivalent of 8 speeds2. Use 2 sets of motors per wheel: one high torque and one high speed. Select between the two. Just like in the Universal Track Unit - https://wiki.opensourceecology.org/wiki/Universal_Track_Unit3. Selecting between motors has low loss if unused motor freewheels 

Cost: 1. Pumps are $100/27 hp or $4/hp2. Motors are $150/15 hp or $10/hp3. Engine costs are $30/hp for throwaway EFI engines at 38 hp - https://wiki.opensourceecology.org/wiki/Engines#1000CC_EFI - but would need to derate engine to access low cost pump (3000 RPM) as engine runs at 3600. Forget about gear reduction. Or do 16 hp - [1] at $25/hp4. Final drive on wheels - ideally direct drive. Otherwise, go to chain - but that's a major failure point for high torque high power stuff outside of skid steers, where chain drive is normal. So practical answer is - gears etc.

Cost - Godzilla engine - https://chatgpt.com/share/6948060e-c8ec-8010-b597-8650a69e938a

No easy answer with hydraulics, really requires some thinking at higher powers. I think we would need to open source our own EFI engine using open source EFI which already exists, and then work out the high torque for wheels. Why do you suggest variable displacement pumps instead of engine throttle? Sure, but cost is $8k. Maybe start with 45 mph top speed heavy truck (8000 lb) that can also be a skid steer.  Suggestion: 1. 4 power cubes, fixed displacement system of 152 hp (4x38 hp). Add electronic controller (arduino, etc) to control throttle and engine turn-on so we have a system that functions close to variable displacement system. 2. Add RTK GPS, computer vision, remote control, autonomous driving - as a way to make up for low top speed.3. Max out, develop, document  limits of above system - commercialize it. I think that  even an 18hp x4 power cube dirt cheap system - with full electronic controls and RTK GPS etc leveraging all modern electronics and AI - could be a killer at low cost. Then we use this to bootstrap open source hardware engines and hydraulics.4. Move on to variable displacement5. Open source an EFI engine6. Open source hydraulics So in the above scenario, if we do 16 hp at $25/hp and 70 lb weight - we do the 'mother cube' concept with a lot of babies - definitely for 4 engines and see where we are at a 64 hp sissy vehicle. Potentially scale by adding another mother power cube combo to double this. Pump - 12 gpm - 3500 rpm - https://www.amazon.com/Hydraulic-Straight-Keyed-Shaft-Aluminum/dp/B07H1HDPBR - so about $10/hp Motor - also similar price - $10/hp So drive cost is $45/hp - so $7k for 150 hp. Just hydraulics are $20/hp, compared to your say $10k/150 hp for variable displacement = $70/hp. Maybe start with 150hp of Power Cube on the back of an old Ford Ranger for $7k? 70 lb per 16 hp - 700 lb for 160 hp. Hard to compete on price until all this becomes open source. Not sure where to go on this. Marcin

Tom First Email

Marcin;

I did some chatting with the AI and came to some conclusions for design and part selection for replacing the transmission of an existing truck with hydraulics.

It incorporates a pump with variable displacement and integrated controls.

The cost estimates do not include the truck.

Hydraulic Truck Design

Below is a complete, end-to-end optimized single-axle hydrostatic propulsion system for a ~4,500 lb truck, designed for 80 MPH top speed, using a mobile-optimized pump (Rexroth A4VG class). This includes component sizing, ratios, flows, pressures, cooling, plumbing, and control logic, plus a clear comparison note to the 4-motor layout at the end.

I’ll keep everything numerically consistent and physically realistic.

1️⃣ Vehicle & Performance Targets (Fixed) Parameter

Value

Vehicle mass

4,500 lb (2,040 kg)

Drive layout

Single driven axle

Tire diameter

30 in (0.76 m)

Tire circumference

2.39 m

Top speed

80 MPH (35.8 m/s)

Gradeability

30% at low speed

Engine

120–150 HP ICE @ ~2,100 RPM

Duty

Continuous road + grade

2️⃣ Wheel Speed & Torque Requirements Wheel speed @ 80 MPH

RPM Wheel RPM=2.3935.8×60≈900 RPM

Wheel torque for 30% grade

F=mg×0.30=2040×9.81×0.30≈6000 N  T=F×r=6000×0.38≈2280 Nm

Add margin → Target = 2500 Nm at wheels

3️⃣ Optimal Pump Selection (Why Not A11VG) ✅ Selected Pump: Rexroth A4VG125 Why

Designed for vehicle propulsion

Higher dynamic efficiency

Faster swash response

Better electronic control support

Rated for continuous mobile duty

Pump Specifications Parameter

Value

Type

Variable displacement axial piston

Displacement

125 cc/rev

Max pressure

400 bar (5800 psi)

Continuous pressure

350 bar

Rated speed

2,500–3,000 RPM

Flow @ 2100 RPM

262 L/min (69 GPM)

Control

Electro-hydraulic (ECU)

4️⃣ Motor Selection (Single-Axle Drive) ✅ Selected Motor Variable axial piston motor (closed-loop)

Parameter

Value

Displacement

180 cc/rev

Max pressure

350–400 bar

Max speed

2,800 RPM

Efficiency

90–93% (typical)

Motor Torque @ 350 bar T=2πΔP×D T=2π35×106×180×10−6≈1000 Nm

5️⃣ Axle Ratio Optimization To hit 80 MPH without overspeeding motor:

Ratio  Axle Ratio=Wheel RPMMotor RPM

Choose motor RPM ≈ 2250 RPM

9002250≈2.5:1

Wheel Torque with Axle Reduction

1000×2.5=2500 Nm

✔ Meets gradeability target ✔ Leaves margin for rolling resistance

6️⃣ System Flow Check (Critical) At 80 MPH:

Motor speed = 2250 RPM

Q=2250×180/1000=405 L/min

But pump max = 262 L/min, so we use motor displacement reduction at speed.

Motor at cruise displacement (~115 cc)

Q=2250×115/1000≈259 L/min

✔ Matches pump capacity ✔ Full displacement only used at low speed / high torque

7️⃣ Complete Hydraulic Schematic (Text) ICE (120–150 HP @ 2100 RPM)

       │
       ▼

Rexroth A4VG125 Variable Pump

 ├─ Electro-hydraulic swashplate control
 ├─ Charge pump (25–30 bar)
 ├─ Cross-port reliefs (380 bar)
 └─ Flushing valve (10–15 L/min)
       ⇄ High-pressure closed loop ⇄

Variable Axial Piston Motor (180 cc)

       ▼

Differential / Axle (2.5:1)

       ▼

Drive Wheels

8️⃣ Charge, Cooling & Filtration (Mandatory) Charge Circuit Item

Spec

Charge pressure

25–30 bar

Charge pump

Internal gear pump

Suction filter

125 µm

Return filter

10 µm

Cooling Hydraulic losses at cruise + grade → ~30–50 kW heat

Component

Size

Oil cooler

40–60 kW

Oil volume

40–60 L total system

9️⃣ Line Sizing Closed-loop lines Velocity target: ≤6 m/s

Flow: ~260 L/min

Line size: 1¼–1½ in ID

Charge & flush ¾ in typical

🔟 Control Strategy (Vehicle-Grade) Recommended Electronic swashplate control

Throttle → pump displacement

Pressure feedback → torque limiting

Engine anti-stall logic

Neutral @ zero displacement (no clutch needed)

Optional:

Wheel speed sensor

Cruise control logic

Traction limiting

1️⃣1️⃣ Performance Summary Metric

Result

Top speed

80 MPH

Gradeability

30%

Launch torque

Excellent

Efficiency

High for hydrostatic

Drivetrain simplicity

No gearbox/clutch

Reliability

Mobile-grade

1️⃣2️⃣ Comparison Note — Why Single-Axle vs 4-Motor Feature

Single-Axle

4-Motor

Complexity

Low

High

Cost

Lower

Higher

Traction control

Moderate

Excellent

Highway efficiency

Better

Lower

Off-road bias

Medium

Strong

For 80 MPH truck use, single-axle hydrostatic is the correct architecture.

✅ Final Recommended Core Components Component

Recommendation

Pump

Rexroth A4VG125

Motor

180 cc variable axial piston

Axle ratio

2.5:1

Engine

120–150 HP ICE

Control

Electronic swashplate + ECU

Cooling

50 kW oil cooler

If you want next I can:

Estimate 0–80 MPH acceleration

Compare fuel economy vs mechanical transmission

Show emergency braking & downhill control

Provide exact Rexroth part numbers

Just tell me where you want to go next.

Please show the cost estimate and the CAD schematic.

ChatGPT said: Absolutely — I’ll provide:

✅ A cost estimate for the optimized single-axle hydrostatic propulsion system, and ✅ A CAD-style schematic (diagram) of the system layout. Before I generate the CAD schematic, I need one preference from you so I can format it correctly:

Which CAD/diagram style do you want? Please choose one of the following:

Industrial P&ID style — Standard hydraulics schematic Looks like a pump/motor/valves diagram with symbols and annotations. Vehicle drivetrain layout — Block diagram showing mechanical and hydraulic connections Great for overall system concept. 3D assembly-style schematic — More engineering drawing feel (simplified 3D parts)

Reply with 1, 2, or 3.

Cost Estimate Summary (2025 US List Prices) These are ballpark ranges. Actual prices vary by vendor, controls, accessories, and installation.

🔧 Core Hydraulic Propulsion Components Component

Qty

Est. Unit Cost

Subtotal

Rexroth A4VG125 variable pump

1

$3,500 – $5,500

$3,500 – $5,500

Variable piston motor (180 cc)

1

$4,000 – $6,000

$4,000 – $6,000

Charge pump (gear, 20–30 bar)

1

$300 – $600

$300 – $600

Flushing valve

1

$250 – $500

$250 – $500

Cross-port relief valves

2

$100 – $200

$200 – $400

Hydraulic cooler + fan

1

$800 – $1,800

$800 – $1,800

Heat exchanger, hoses, reservoirs

—

$800 – $1,500

$800 – $1,500

Subtotal – Hydraulic components: ➡️ ~$9,850 – $16,300

🧠 Controls & Sensors Item

Qty

Est. Unit Cost

Subtotal

ECU / pump controller

1

$600 – $1,200

$600 – $1,200

Electronic swash control valve

1

$400 – $900

$400 – $900

Pressure sensors (HP & charge)

2

$75 – $150

$150 – $300

Motor speed sensor

1

$100 – $250

$100 – $250

Wiring harness/canbus interface

—

$200 – $600

$200 – $600

Subtotal – Controls: ➡️ ~$1,450 – $3,250

🔩 Misc Hardware & Fabrication Item

Est. Cost

Mounting brackets & gear adapters

$300 – $900

Differential/axle adaptation

$400 – $1,200

Hoses, fittings, filters

$400 – $1,000

Labor (hydraulic plumbing)

$800 – $2,000

➡️ ~$1,900 – $5,100

⭐ Total Estimated System Cost Low end:

$13,200

High end:

$24,650

👉 Most realistic builds land around:

$16,000–$20,000

(This excludes vehicle chassis cost and engine — calculated for the hydrostatic drive subsystem only.)

Questions for the CAD Schematic Reply with 1, 2, or 3 based on the diagram style you want:

1 ➜ Industrial P&ID schematic 2 ➜ Vehicle drivetrain block diagram 3 ➜ Simplified 3D assembly view Example reply: ➡ “Show style 2”

Once you pick the style, I’ll generate the CAD-like diagram instantly.

You said: 2

ChatGPT said: Perfect — below is a clean vehicle-drivetrain (Style 2) CAD-style schematic, plus a refined cost estimate tied directly to the components shown.

I’ll present this the way it would appear in a concept CAD / system integration drawing, just expressed textually.

🚗 Single-Axle Hydrostatic Drivetrain (Vehicle-Layout CAD Schematic – Concept Level) 📐 System Overview (Block / Layout Diagram)

┌───────────────────┐
│   ICE ENGINE      │
│ 120–150 HP @2100  │
└─────────┬─────────┘
          │  (SAE bellhousing / coupler)
          ▼
┌───────────────────────────┐
│ REXROTH A4VG125 PUMP      │
│ Variable Displacement     │
│                           │
│  • Electronic swash ctrl  │◀── Throttle / ECU
│  • Charge pump (25–30 bar)│
│  • Cross-port reliefs     │
│  • Flushing valve         │
└───────┬─────────┬─────────┘
        │ A-line   │ B-line   (Closed high-pressure loop)
        │          │
┌───────▼──────────▼───────────┐
│  VARIABLE AXIAL PISTON MOTOR  │
│      ~180 cc / rev            │
│                               │
│  • High torque @ low speed    │
│  • Destrokes at cruise        │
└──────────┬────────────────────┘
           │  (Mechanical shaft)
           ▼
┌───────────────────────────┐
│ DIFFERENTIAL / FINAL DRIVE│
│        Ratio ≈ 2.5 : 1    │
└──────────┬─────────┬─────┘
           │         │
       ┌───▼───┐ ┌───▼───┐
       │ WHEEL │ │ WHEEL │
       │  LHS  │ │  RHS  │
       └───────┘ └───────┘

🔁 Support Circuits (Shown Separately in CAD) Charge Circuit: ────────────── Pump Charge Port

       │
       ▼
  Suction Filter (125 µm)
       │
  Charge Pump
       │
  25–30 bar Regulator
       │
  Loop Check Valves
       │
  Main Closed Loop

Cooling / Flush: ─────────────── Loop → Flushing Valve (10–15 L/min)

       │
  Oil Cooler (≈50 kW)
       │
  Return Filter (10 µm)
       │
  Reservoir

⚙️ Key Design Numbers (As Annotated in CAD) Item

Value

Wheel speed @ 80 MPH

~900 RPM

Motor speed @ cruise

~2250 RPM

Motor displacement

180 cc (low speed) → ~115 cc (cruise)

Pump displacement

0 → 125 cc

System pressure

150–220 bar cruise / 350 bar max

Wheel torque (max)

~2500 Nm

Top speed

80 MPH

💰 Cost Estimate (Mapped to Diagram) 🔧 Core Propulsion Hardware Component (shown above)

Qty

Cost (USD)

Rexroth A4VG125 pump

1

$3,500 – $5,500

Variable axial motor ~180 cc

1

$4,000 – $6,000

Charge pump (gear)

1

$300 – $600

Cross-port relief valves

2

$200 – $400

Flushing valve

1

$250 – $500

Differential / axle mods

1

$400 – $1,200

Subtotal (propulsion): ➡ $8,650 – $14,200

❄ Cooling, Plumbing & Filtration Item

Cost

Oil cooler + fan (≈50 kW)

$800 – $1,800

Filters (suction + return)

$150 – $350

Hoses, fittings, hard lines

$500 – $1,200

Reservoir (40–60 L)

$300 – $700

Subtotal: ➡ $1,750 – $4,050

🧠 Controls & Electronics Item

Cost

ECU / pump controller

$600 – $1,200

Electronic swash valve

$400 – $900

Pressure & speed sensors

$250 – $600

Wiring / CAN harness

$200 – $600

Subtotal: ➡ $1,450 – $3,300

🔩 Fabrication & Integration Item

Cost

Engine–pump adapter

$300 – $800

Motor mount & brackets

$300 – $900

Labor (hydraulics + setup)

$800 – $2,000

Subtotal: ➡ $1,400 – $3,700

✅ Total System Cost (Realistic) Estimate

Cost

Low end

≈ $13,300

Typical build

≈ $16,000 – $19,000

High end

≈ $25,000

(Engine, chassis, and wheels not included — drivetrain only.)

🏁 Engineering Verdict For a 4,500-lb truck @ 80 MPH, this system:

✔ Uses vehicle-optimized hydraulics ✔ Eliminates gearbox and clutch ✔ Delivers high launch torque + highway speed ✔ Is simpler and more efficient than 4-wheel hydraulic drive ✔ Is realistically buildable with off-the-shelf components

If you want next I can:

Convert this into a dimensioned CAD drawing (DXF-style layout)

Estimate fuel economy vs automatic transmission

Model 0–80 MPH acceleration time

Compare against electric drivetrain costs

Just tell me which one you want.

You said: Estimate fuel economy vs automatic transmission

ChatGPT said: Below is a realistic, engineering-based fuel-economy comparison between:

Your hydrostatic drivetrain (A4VG + motor, single axle) vs A conventional automatic transmission (torque-converter + gears)

for a 4,500 lb truck, 80 MPH capable, 120–150 HP ICE.

I’ll be very explicit about where losses come from and when hydraulics make sense.

1️⃣ Baseline Assumptions (Same for Both) Parameter

Value

Vehicle mass

4,500 lb

Engine

120–150 HP gasoline or diesel

Cruise speed

65–70 MPH

Driving

Mixed highway + moderate grades

Tires

~30 in

Aerodynamics

Truck-like (Cd ~0.45–0.55)

2️⃣ Drivetrain Efficiency (Key Difference) 🔧 Automatic Transmission (Modern 6–8 speed) Component

Efficiency

Torque converter (locked)

97–99%

Gearsets

96–98%

Differential

95–97%

Total drivetrain

90–94%

Once locked up in top gear, losses are very low.

⚙️ Hydrostatic Drivetrain (Optimized, Closed-Loop) Component

Efficiency

Variable pump

90–92%

Variable motor

90–92%

Closed loop plumbing

97–98%

Final drive

95–97%

Total drivetrain

75–82%

Even with excellent components, hydraulic energy conversion costs you efficiency.

3️⃣ Fuel Economy Comparison (Highway Cruise) Assume engine needs ~45 HP at 65–70 MPH (typical for this vehicle)

🚙 Automatic Transmission

  from engine45 HP/0.92≈49 HP from engine

Gasoline BSFC ≈ 0.50 lb/hp-hr

Fuel flow:

49×0.50=24.5 lb/hr

Gasoline ≈ 6.1 lb/gal

24.5/6.1≈4.0 gal/hr

At 68 MPH:

17–18 MPG

🚜 Hydrostatic Drive

  from engine45 HP/0.78≈58 HP from engine

Fuel flow:

58×0.50=29 lb/hr  29/6.1≈4.75 gal/hr

At 68 MPH:

14–15 MPG

4️⃣ Summary Table (Cruise) Drivetrain

MPG @ 65–70 MPH

Automatic transmission

17–18 MPG

Hydrostatic (this design)

14–15 MPG

Difference

−15 to −25%

5️⃣ Where Hydrostatic LOSES ❌ Continuous energy conversion (mechanical → hydraulic → mechanical) ❌ Pump & motor leakage at low torque cruise ❌ Heat rejection (oil cooler load) ❌ Less benefit from steady-state gearing This is why hydrostatic drives are rare in highway vehicles.

6️⃣ Where Hydrostatic WINS Hydraulics can be better or equal when:

✅ Low-speed / stop-start duty ✅ Frequent grade changes ✅ Creeping, towing, off-road ✅ No gear hunting ✅ No torque-converter slip ✅ High launch torque without large engine In urban / off-road duty, MPG difference can shrink to 5–10%, sometimes favoring hydraulics.

7️⃣ Acceleration & Driveability Comparison Feature

Automatic

Hydrostatic

Launch torque

Good

Excellent

Smoothness

Good

Excellent

Gear shifts

Yes

None

Engine lugging

Possible

Prevented by control

Hill control

Good

Excellent

8️⃣ Can Hydrostatic MPG Be Improved? Yes — but never equal to a mechanical gearbox.

Improvements (Realistic Gains) Fixed motor + multi-speed axle: +5–8%

Smaller pump + higher pressure: +3–5%

Cruise destroking optimization: +2–3%

Diesel engine instead of gasoline: +15–20% overall

Even then, expect:

Best case hydrostatic = ~85% of automatic MPG

🏁 Final Engineering Verdict For a 4,500 lb truck @ 80 MPH: Automatic transmission ✔ Best fuel economy ✔ Simpler cooling ✔ Lowest highway losses Hydrostatic drive (your design) ✔ Superior low-speed control ✔ Outstanding grade & towing performance ✔ Mechanically simpler drivetrain ❌ ~15–25% fuel economy penalty at highway cruise

Bottom line: Hydrostatic propulsion is justified if your priority is control, torque, and off-road performance — not maximum MPG. If you want, I can:

Model city vs highway MPG split

Estimate towing fuel economy

Show a hybrid (hydraulic + mechanical) compromise

Compare against electric drivetrain efficiency

Top Speed

• 150 hp gives top speed of 60 mph at 7% incline - [2]